No KO is an unofficial term for a custom stage created in Stage Builder that is surrounded by objects which cannot be passed through, making it generally impossible to be KO'd or self-destruct. These types of stages provide a different twist on the normal setup, as players fight continuously in an inescapable area, without re-spawning and with increasingly high damage percentages. These types of stages can in general be used for matches with various externally generated goals, such as being the first to reach a certain amount of damage, or having the least damage at the end of a time match, and are also conducive to the more traditional Stamina Mode. The only ways to "win" a battle on a No KO stage is to be in Stamina or Coin mode (unless it ends in Sudden Death). A No KO stage might also be used simply for sparring without the need for recovery.
A standard/typical No KO stage is composed of a square/rectangular area of basic blocks, with spikes (in Brawl) or lava (in for Wii U and Ultimate) along the top to prevent characters from spawning there. Variations include a triangular shape to cause a slant for items, hindering item usage as soon as they spawn, a more rounded stage to provide an aiming challenge, a stage made of icy blocks, and a roof of drop blocks.
A box is a common type of No KO stage. It is impossible to get knocked out of a box due to its design. There are two main ways to build a box. The first is to simply build a stage-builder stage and make the walls and bottoms of the stage sealed by blocks. Then seal the "top" of the box with spikes and falling blocks. The other method is to have the top, left and right sides sealed by blocks and the bottom as drop through ledges, allowing combatants to enter from the bottom and not be able to leave accidentally. In order to make this easier, another strip underneath can be used to land on before jumping in.
As the stage requires at least one space between the stage border and the blast line, there are several Final Smashes that allow players to cross this barrier and be KO'd. Ike's Great Aether, Greninja's Secret Ninja Attack, Robin's Pair Up, Cloud's Omnislash, and the Mii Brawler's Omega Blitz send the user and the target to the peak of the stage, causing a Star KO outside of the main play area. Lucario's Aura Storm and Bowser Jr.'s Shadow Mario Paint also sends their users to the peak, allowing them to escape, and Pikachu's Volt Tackle in Brawl and Smash 4 allows passage through solid objects and may culminate outside of the intended arena. Snake's Grenade Launcher, Olimar's End of Day, and Zero Suit Samus' Gunship in for Wii U also put them to the stage peak after the Final Smash is over as they come back down from the top of the screen. All three Star Fox characters' Landmasters will put them at the top as well. Bayonetta's Infernal Climax, Ridley's Plasma Scream, Zelda's Triforce of Wisdom, Bowser's Giga Bowser Punch, Joker's All-Out Attack, and Sora's Sealing the Keyhole, if successful, will KO the opponent without regard for the blast lines so long as their damage is high enough. Outside of Final Smashes, it is also possible to escape using a Warp Star or if all three Dragoon pieces are collected and used. Death's Scythe will trigger an instant KO if the enemy's damage is high enough. Hero's Whack and Thwack commands can cause an instant KO to an opponent and the Kamikaze command will KO himself.
In certain situations, a glitch may cause a fighter to be pushed or launched through a wall. In Brawl, a high enough launch speed can cause this, while in SSB4, characters are typically unable to pass through even very thin terrain no matter how fast they move, requiring more exotic arrangements of terrain in order for clipping to occur.