A ladder is a technique usable by a team of two players using the same character that allows them both to attain normally unreachable heights.
The Luigi ladder involves two Luigis repeatedly using their sourspotted Super Jump Punch against each other back and forth, allowing them to steadily climb upwards and away from the stage. An applied usage allows players to stall indefinitely against a ceiling.
The technique can be performed in every game in the series, though its applications can be varied. In Super Smash Bros., Super Jump Punch grants some horizontal distance, allowing to act as a method for stage jumping.
The technique's use, however, is most infamous in Melee. Changes to Luigi's Super Jump Punch causes the technique to instead send the two Luigis straight upward, and the team can even jump up beyond the upper blast line. Applied usage of the technique can also allow for recovery options, though the lack of horizontal distance from Super Jump Punch makes this a situational use. The Luigis do not KO each other since Super Jump Punch has no knockback. Similar variants of the technique involving attacks like Link's Spin Attack and Samus' Screw Attack end up KOing the characters when they cross the blast line.
While carried over for Brawl, the two Luigis now KO each other upon crossing the upper blast line; despite this, it can be used for stalling against a ceiling. This is retained in Smash 4.
In Ultimate, like Melee, the Luigi ladder does not KO at the upper blast line, however, this property invokes a glitch. As the two Luigi's continually rise, the character markers on the edge of the screen start to leave the middle of the top of the screen and start going to the sides. Once they reaching the sides, they go to the bottom of the screen and begin to come to the middle as the Luigis rise. Adding to that, the character markers show the many sprites of the Luigis as they rise. In addition to this, if one or both Luigis are on their last stock, the Finish Zoom effect will constantly play after the Luigis reach the upper blast line, however neither of the Luigis will get KOed.
The Pac-Ladder involves two Pac-Mans repeatedly using their up special move, Pac-Jump, alternately. As each Pac-Man bounces on the other's trampoline, they both climb successively higher until they are both out of reach for many opponents. They can then stall with the technique until time ends. If the match is decided by Sudden Death, then the process may be repeated if needed until Bob-ombs eventually spawn under the Pac-Man players and fall onto their opponents. Team attack must be turned off for this ladder.
The Marth ladder involves two Marths repeatedly using their up special move, Dolphin Slash, alternately. As each Marth gets hit by the other's attack, they both climb successively higher until they are both out of reach for many opponents.
The Yoshi ladder is usable in Melee, performed by using Yoshi's Egg Lay on another Yoshi. After the first Yoshi escapes the egg, he regains his midair jump, uses Egg Lay on the other Yoshi, immediately uses the midair jump, and the cycle can then repeat. This technique was popularised by aMSa with fellow Yoshi main Plata.
The Shulk ladder functions similarly to the Marth ladder, with two Shulks repeatedly using Air Slash one after the other and canceling helplessness using the other player's attack. As Air Slash's first hit has set knockback, this can be done indefinitely, and Shulk's Monado Arts allow him to achieve this in multiple ways. However, care must be taken not to use the second slash, which has actual KO potential. It is also difficult to do in the Smash Art, since knockback given and taken is increased.
The Cloud ladder functions similarly to the Shulk ladder, with two Clouds repeatedly using Climhazzard one after the other and canceling helplessness using the other player's attack. However, the laddering players must be wary of their Limit, as this technique will slowly charge each player's Limit Gauge. If a player fully charges their Limit Gauge, it is possible to preserve the ladder by waiting a moment and falling a bit before using Limit Climhazzard; however, at high percents, a rising hit will KO the other Cloud. It is possible to continue laddering after a Limit Climhazzard under a stage like Smashville or a wall-less Ω form stage if the attacked Cloud successfully performs a tech on the ceiling, but this is not safe: not only could the Cloud miss the tech, but many characters can access those areas of stages and interrupt the ladder without risk of losing a stock, such as with Sheik's Bouncing Fish or with a straight-flying projectile like Link's Boomerang.
Another option would be for the Limit Cloud to fast fall until he reaches a safe distance, where he can perform Limit Blade Beam or Limit Cross Slash while the other Cloud falls normally and awaits a normal Climhazzard. However, the amount of distance the Cloud players fall during this period of time will definitely leave them open to attack.
Ultimately, this ladder is not as consistent in the long run as others.
This is similar to the Pac-Ladder. One Mega Man player uses Rush Coil, launching them and an adjacent teammate into the air. The one who used the up special must jump up to the same height as the other player, who then uses their Coil, restoring the double jump and up special of the first player. Because Rush Coil has no hitboxes, this doesn't require Team Attack to be switched off or on, and never damages the players using the ladder. This, combined with its quick height gain compared to most other ladders, makes it arguably the most effective. An video demonstrating this ladder can be found here.
As stalling is banned in tournaments, ladders are banned in tournament play and are sometimes specifically mentioned in tournament rulesets. Otherwise, it would be possible in doubles (with the usual Friendly Fire turned on) for a team to take an early lead, and then use the ladder technique to create an effectively unbeatable stalling strategy.