Y button

From SmashWiki, the Super Smash Bros. wiki
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The Y button refers to the grey, kidney-shaped button on the right side of the GameCube controller, the small button varying in color on the right side of the Classic controller, Wii U Pro Controller, Wii U GamePad, and Nintendo 3DS, or the dark gray button on the Nintendo Switch Joy-Con and Pro Controller. Like the X button, it is used for jumping for the all games of the Smash Bros. series (including Super Smash Bros. for Virtual Console). When using Joy-Con (R) as a single Joy-Con held horizontally, the Y button will become the top button, but will still perform jumping like the X button.