Priority

Priority is the system that governs how opposing attacks interact with each other in the  series. Most commonly, when the hitboxes of two characters' attacks collide with each other, they clank, freezing both in place for a moment. If the attacks are roughly equal in strength - the difference in damage is 9% or less - then both characters rebound from the impact and pause for a moment before they can do anything else. If one attack is much stronger than the other, it wins outright, and more often than not will continue on to strike the enemy stuck rebounding. The most common exception is aerial attacks, which usually can clank with projectiles, but not with ground attacks or other aerials, and cannot rebound at all. Certain hitboxes are also marked as being "cannot clank" or "cannot rebound", affecting their ability to cancel out (and be cancelled out by) opposing moves.

While the effects of priority are calculated based on individual hitboxes, the results are applied to the attacker as a whole. For example, if one hitbox clanks with an enemy attack, the entire move clanks, regardless of how many other hitboxes would not have clanked. In addition, an attack that clanks is still dangerous to bystanders until rebound begins, even if it was entirely outprioritized by the opposing attack.

The term "priority" is often informally used to describe a move's general ability to beat other moves, due to factors such as its damage output, hitbox position, attack speed, non-vulnerable hurtboxes, or other properties.

Normal priority
Normal priority describes the set of rules that apply to normal attack hitboxes, that is, any attack hitbox that does not have a special property such as transcendent or trample. As such, the majority of standard attacks, aerials, special attacks, and projectiles contain hitboxes with normal priority.

Ground attacks
When two ground attack hitboxes overlap, they will clank. This collision is signified by a white "bubble", and in Melee, a distinct "ting" sound, as if swords are clanging, is also heard. If two actual swords clank, such as Link and Marth's blades, a more realistic "ting" sound is heard. If the stronger hitbox (by %) deals more than 9% (the "priority range") more than the weaker hitbox, the stronger move will continue as normal and the weaker move will end, with the character going into rebound. If both hitboxes deal within 9% of each other, both moves will end and both characters will go into rebound. (note that in the original Super Smash Bros., the priority range is instead 10%). This comparison happens after most damage multipliers but before the 1v1 multiplier in Ultimate.

In Melee, the weaker hitbox won't hit the opponent if it clanks. However, the stronger hit can still hit the user of the weaker hit immediately, or on the following frames. In Ultimate, however, neither player can be hit by the clanking opponent on the frame that the clank occurs. If the stronger move continues, it can hit the clanking opponent on the following frame, assuming the move still has a hitbox on that frame.

For example:
 * In Brawl, if 's un-tipped forward smash collides with 's forward smash (14% vs. 22%), both moves will end and both characters will go through rebound.
 * However, if Marth's un-tipped forward smash collides with 's forward smash (14% vs. 24%), Ganondorf's forward smash will continue and may hit Marth as Marth rebounds.
 * If 's forward smash collides with ' up tilt (19% vs. 7%), Captain Falcon's forward smash will continue and hit Ness as Ness starts to rebound.

Rebound
Depending on the relative strength of the ground moves, either one or both characters will go through rebound.

First, a rebounding character will suffer freeze frames equal to the hitlag of the stronger attack (by %). This is true whether the user of the stronger attack goes through rebound or not. During these freeze frames, all hitboxes are disabled. This takes into account all hitlag modifiers, with one exception: the electric modifier is applied based on whether the character's hitbox has the electric effect. So if Pichu's ftilt clanks with Chrom's ftilt, Chrom will be frozen for 13 frames (typical for Chrom ftilt, which is the stronger move), but Pichu will be frozen for 19 frames (equivalent to hitlag that Chrom's ftilt would have if it had the electric effect).

After the freeze frames are over, characters go through a unique rebound animation.

Prior to Ultimate, the length of the rebound animation increased proportionately to the damage each hitbox would have dealt. This meant that the fighter that performed the weaker attack would get out of rebound before the other and have frame advantage. The calculation for rebound duration depends on the game, but "R" always means rebound frames and "d" always means damage dealt, rounded down. In Melee, the calculation is roughly, except for Dr. Mario, for which it is   instead. In Smash 4, the calculation is.

In Ultimate, the length of the rebound animation for both characters is based on the damage dealt by the stronger hitbox (again, regardless of whether the user of the stronger move is going through rebound or not). It uses the same formula as Smash 4, although the maximum rebound is 58 frames.

Overall, this means that, in Ultimate, if both characters go through rebound they will be actionable on the same frame. The exception is if one character clanked using an electric move; that character will be actionable a bit later than the opponent due to the extra freeze frames.

If only one character goes through rebound, they will often be hit by the opponent's move as it continues. However, this is not necessarily the case if the original move was sufficiently disjointed or if the clank happens on the last frame of the opponent's hitbox. In that case the opponent will suffer the end lag of their attack whereas the character will suffer the freeze frames plus rebound animation, with the length of both based on the opponent's move.

Projectiles
Similar rules apply when ground attack hitboxes overlap normal projectile hitboxes. If the ground move deals more than 9% more than the projectile, the projectile is destroyed and the ground move continues uninterrupted. If the two moves deal within 9% of each other, the projectile is destroyed and the ground move is canceled into rebound. If the projectile deals at least 9% more than the ground hitbox, the ground move is canceled into rebound and the projectile continues (usually hitting the opponent on the following frame).

Similarly, if two projectiles collide, the projectiles will clank and the weaker projectile will be destroyed. The stronger projectile will either continue or also be destroyed depending on whether it does at least 9% more than the weaker projectile.

Note that there are some projectiles that don't get destroyed by clanking. However, their hitboxes are generally disabled for the character or object that clanked with them.

For example:
 * Marth's un-tipped forward smash is out-prioritised by ' fully charged Charge Shot (14% vs. 25%).
 * However, if the second hitbox of 's forward smash collides with Samus' fully charged Charge Shot, they will cancel each other out (23% vs. 25%).

While most normal projectiles follow the rules of ground priority, Snake's Remote Missile, Diddy Kong's Peanut Popgun, and King Dedede's Waddle Dee Toss behave differently. While the 9% range does apply to Dedede's Gordo, its hitbox behaves like an airborne attack as it cannot be cancelled out. For example, if Captain Falcon's forward smash collides with a Gordo (19% vs. 23%), the clash bubble will appear, and the forward smash will be cancelled out while the Gordo is not. However, since both hitboxes are within the priority range, the Gordo will go right through Captain Falcon without harming him, and continue along its natural path. It will, however, still possess its damaging hitboxes, and can still harm any other character it happens to touch along its route.

Snake's Remote Missile has a damagable hitbox and behaves like a thrown item, such as a capsule. Hence, when opposing hitboxes collide with the Remote Missile, they will damage it instead of colliding with it. If the Remote Missile takes enough damage, it will be destroyed. The peanuts from Diddy Kong's Peanut Popgun behave the same way, but any colliding hitbox, even if it only deals 1%, will destroy the peanuts.

Aerial attacks
Different rules apply to the hitboxes of normal aerial attacks. When a normal aerial attack hitbox overlaps that of a normal ground attack or another normal aerial, the attacks do not clank. If an overlap between the hitboxes of two such attacks occurs, both will persist, irrespective of each other, and will damage their foes if they contact them.

For example:
 * If 's back aerial overlaps 's up tilt, neither attack is cancelled, and both characters are hit by each other's attacks.

However, the hitboxes of aerial attacks can clank with projectiles. The aerial will suffer freeze frames based on the stronger move as described above, but then the aerial will continue. The projectile will be destroyed unless it deals more than 9% more than the aerial attack. In that case it is likely to hit the aerial user on the following frame.

For example:
 * If Marth's forward aerial collides with Samus' fully charged Charge Shot (10% vs. 25%), his forward aerial will not be cancelled out, but it will fail to cancel out the Charge Shot. As a result, the collision action will occur, but Samus' Charge Shot will out-prioritise the forward aerial and hit Marth.
 * However, if Ganondorf's forward aerial collides with Samus' fully charged Charge Shot (17% vs. 25%), his forward aerial will cancel out the Charge Shot and Ganondorf will not be harmed.

Grabs
Grabboxes are not bound by usual priority rules. Instead, in each game, they behave differently if a grab connects with an opponent at the same time the opponent strikes the grabber with a standard hitbox.


 * In Smash 64 and Melee, the grab has full priority over the hitbox, with the grabber ignoring it entirely and hitting the opponent.
 * In Brawl, the grab has priority over the hitbox, but the hitbox still causes damage to the grabber, instead having them armor through it.
 * In Smash 4 and Ultimate, hitboxes outprioritize grabs; in this situation, the grabber receives full damage and knockback from the standard hitbox, while the attacker is grab released and takes 3% damage (or more if the grab came from a special move and the grab itself dealt damage, like the beginning of Falcon Dive). In Ultimate, however, a select few attacks' hitboxes are coded to be outprioritized by grabs, with the grabber taking damage but armoring through the attack like in Brawl; this is the case for 's Homing Attack, Spin Dash, and Spin Charge, and more notably 's Wonderwing (which also causes the grabber to take 0.25× damage from the hitbox).

Likewise, if both characters attempt to grab each other at the same time, the situation is resolved differently in each game. In Smash 64, Melee, and Brawl, this is determined by port priority, with the character in the lower port always getting the grab. In Smash 4, port priority is no longer a factor, with either character having an equally random chance to get the grab instead. Ultimate resolves both quirks by simply causing both grabs to cancel each other out, akin to a clang. This property is known as grab parrying by the in-game tips, and results in both characters taking 1% damage and performing their rebound animation (unless they are in the air, where their grab release animation is used instead). The exact timing of when this can happen is specific to every animation of every character, and may even be assigned to actions that are not grabs.

Transcendent priority
Transcendent priority (also known as transcending priority) refers to hitboxes that cannot clank with other hitboxes, meaning they won't cancel out, or be cancelled out by, other hitboxes, even other transcendent hitboxes.

Transcendent grounded moves cannot clank with other grounded moves, meaning that they are more prone to trading. They are also unable to cancel out projectiles.

Transcendent projectiles will go through other projectiles as if they're not there, and can't clank with aerials or grounded moves (meaning that they will generally hit the opponent if the opponent attempts to clank).

In Smash 64, transcendent priority only existed for certain projectiles (such as 's ) but from Melee onward, it could exist for any kind of attack.

For example:
 * Brawl Meta Knight's forward smash is transcendent. If a pair of Meta Knights use their forward smash at the same time, and the forward smash hitboxes overlap each other, they will not collide, and their animations and damaging properties will continue until the attack ends naturally. If the two Meta Knights are close enough together, each will be hit by the other's forward smash.
 * Lucas' forward smash is transcendent. If Donkey Kong's forward smash overlaps with Lucas' forward smash (20% vs. 15%), Lucas' forward smash will go through Donkey Kong's without colliding. If its hitboxes touch Donkey Kong, he will receive damage and knockback from Lucas' forward smash, while Lucas will be hit by Donkey Kong's forward smash as well since normal hitboxes will go through transcendent hitboxes.
 * As of version 9.0.0, many rapid jabs and their finishers in Ultimate are transcendent. This is to ensure that the final hit will connect without the fear of an opponent canceling the move with their own jab.

Moves that cannot rebound
Some attacks cannot ever rebound (also known as trampling); in other words, they will continue their animations as if they did not collide at all, but the hitbox will count as blocked for other clanking character, as long as the the trampling move was not more than 9% stronger. However, if the trampling move has multiple hits, then later hits can still connect as clanking blocks only the current hit. This can be advantageous for moves with this property that are either multihitting in nature, very fast, or very damaging (which would increase the length and therefore vulnerability of the rebounding animation), but is disadvantageous if the move's endlag is longer than the length of the opponent's freeze frames plus rebounding animation. Additionally, two moves that cannot rebound that are used against each other are more prone to damaging both characters instead of cancelling out their hitboxes.

Strangely in Brawl, the hitbox flag which should determine whether a move can rebound or not is discarded as soon as the hitbox is generated. This means that moves in Brawl which are not supposed to rebound (such as ) will rebound anyway. This was most likely a coding error.

* Should not rebound but rebounds anyway, due to the anti-rebound flag not working properly in Brawl.

** Does not rebound in later games, but the effect is nullified by the move gaining transcendent priority.

Considered both airborne and grounded
*The second hitbox of Zero Suit Samus' up smash behaves like an aerial. It may be out-prioritised by another character's aerial, but the up smash's animation and subsequent hitboxes cannot be cancelled.

The hitboxes of the above attacks can collide with the hitboxes of other attacks regardless of if they're ground or aerial attacks. When such a collision occurs, the clash bubble will appear and the outcome will follow the law of high and low priority. However, airborne attacks cannot be cancelled out, including airborne special moves. Thus, while the results of a collision will follow the law of high and low priority, the appearance of such a collision will differ to that of two colliding ground attacks.

If one of the above ground attacks collides with an aerial, or one of the above aerials collides with a ground attack, and the ground attack is below the priority range of the aerial, the ground attack will be cancelled and the airborne attack can damage the wielder of the ground attack. If the two hitboxes are within the priority range, the ground attack will be cancelled out, and, since it is an aerial attack, the animation of the airborne attack will persist. However, the airborne hitbox becomes harmless, and the wielder of the ground attack will not at all be knocked or damaged by the airborne hitbox, regardless of whether it connects or not.

For example:
 * If King's Dedede up tilt collides with Meta Knight's glide attack (13% vs. 13%), Dedede's up tilt will be cancelled out, but he will not be harmed, even if the hitbox of Meta Knight's glide attack contacts King Dedede's body.

If the hitbox of a ground attack is above the priority range of a colliding airborne hitbox, both attack animations will persist, but the airborne hitbox will become harmless, while the ground attack is still able to damage. Thus, if the hitbox of the ground attack connects with the body of the character who used the airborne special, this character will take the damage and knockback of the ground attack. A notable example of a special move that can collide with the hitboxes of other attacks while airborne is 's Mach Tornado. If performed on the ground (grounded), it can be cancelled out, but if airborne, it cannot be cancelled. Mach Tornado is a multi-hit move that deals 1% per hit during the main portion of the attack.

For example:
 * If Captain Falcon's forward tilt collides with a hitbox of Meta Knight's grounded Mach Tornado (10% vs. 1%), the forward tilt will collide with, and cancel out the Mach Tornado.
 * If the Mach Tornado is airborne, Captain Falcon's forward tilt can cancel one of its hitboxes, but cannot cancel the hitboxes that follow. Mach Tornado cannot cancel his forward tilt either, because it is too weak (10% vs. 1%). However, unless the hitbox of Captain Falcon's forward tilt connects with Meta Knight's body, the Mach Tornado will continue and can afterwards damage Captain Falcon, despite the fact that his forward tilt out-prioritised it.

Non-character objects with complex animations
A select few attacks in SSB4 will collide with other attacks, but can only nullify projectiles within the 9% range and will always lose a trade with regular hitboxes regardless of the 9% rule, much like a grab. For example, Bayonetta's up smash cannot be used as effectively as other up smashes to hit opponents in the air, as any hitbox that comes into contact with it will render the up smash inert, and if the hitbox is close enough to Bayonetta at the moment of colliding, only Bayonetta will take damage. However, if the opposing hitbox is a projectile or another attack with this property (i.e. Lucario's Aura Sphere or another Bayonetta's up smash), both characters will take damage as if one of them used an aerial hitbox (but the collision bubble will still appear). The overarching pattern among these attacks is that they are character-specific props that feature complex animations beyond simply moving or rotating, and thus it is possible (but not proven) that the game short-sightedly does not attempt to make them capable of trading with attacks from other animating elements (i.e. characters).

Because they cannot override regular hitboxes and it is not always clear to players whether a hitbox is attached to a character or a prop, these attacks are often perceived as having "low priority", and some moveset repositories such as Kurogane Hammer list them under such term.

Trivia

 * In Melee and Ultimate, a unique metallic sound effect can be heard when hitboxes with a slash effect clang with each other.