X button

The X button refers to the grey, cashew-shaped button on the right side of the GameCube controller, the top button of the Classic Controller, Nintendo 3DS, Wii U GamePad, and Wii U Pro Controller, and the top dark gray on both the right Joy-Con and Nintendo Switch Pro Controller. The color of the X button varies on the 3DS and matches the color of the shell on the Classic Controller, Wii U GamePad, and the Wii U Pro Controller.

Overview
Due to the Nintendo 64 controller not having an X button, there is no function for it in the original Super Smash Bros.. However, its Wii Virtual Console re-release made it compatible with controllers that have an X button, and it can perform any action in that state. In Super Smash Bros. Melee, Super Smash Bros. Brawl, Super Smash Bros. 4, and Super Smash Bros. Ultimate, the X button is primarily used for jumping, a function it typically shares with the Y button. In Melee, Brawl, and Smash 4, it also changes the palette swap on the character selection screen. When using the right Joy-Con by itself held horizontally, the X button will become the right button, acting like an A button and performing standard attacks, while the X button's typical functionalities are given to the Y button. The equivalent to this model on the left Joy-con would be the down directional button.

In Brawl, and Smash 4, and Ultimate, the X button can be customized in the controls menu to perform other functions. Nintendo Switch hardware version 10.0.0 introduced the ability to map the functionality of any button to any other button from the console options menu, further diversifying how the X button can be used in Ultimate.