From the Super Mario Wiki
Dotted-Line Blocks (originally formatted as Dotted Line Blocks) are transparent objects found in several Mario games such as Super Mario World, Super Mario World 2: Yoshi's Island, Super Mario World: Super Mario Advance 2, Yoshi's Island: Super Mario Advance 3, Yoshi's Island DS, New Super Mario Bros. Wii and Super Mario Run. They represent that they can be solid when a switch is triggered rather than blocks appearing from nothing. In Super Mario World, they come in four colors and turn into ! Blocks once the player hits the ! Switch in the Switch Palace of the corresponding color. In Super Mario World 2: Yoshi's Island, Yoshi's Island DS, Yoshi's New Island, and Yoshi's Woolly World, after hitting a nearby ! Switch, the Dotted-Line Blocks will temporarily become ! Blocks and are able to be used as platforms. Dotted-Line Blocks in New Super Mario Bros. Wii can turn into and revert from red blocks after pressing the red ! Switches in World 3. In New Super Mario Bros. and Super Mario Run, red (yellow in Run) ! Switches can change Dotted-Line Blocks into red blocks for a limited time.
Transparent boxes similar to Dotted-Line Blocks appear in Super Mario 64 and its remake Super Mario 64 DS, where they turn solid and are able to be used after hitting the ! or ? Switches in the Tower of the Wing Cap in both versions, and in the N64 version only, Vanish Cap Under the Moat, and Cavern of the Metal Cap.
In Super Mario Maker 2, they can be triggered by ON/OFF Switches, with only the red blocks becoming solid when they are on, and only the blue blocks becoming solid when they are off. In the Super Mario Bros. 3 style, they are depicted having a smiling face, while every other style depicts them as faceless blocks.
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