From the Super Mario Wiki
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Semisolid Platforms are platforms found throughout the Super Mario series that players can jump through from the bottom. They have a different design in each installment, first appearing in Super Mario Bros. 2. Another kind of Semisolid Platform is the Mushroom Platform.
Super Mario series
Super Mario Bros. 2
In Super Mario Bros. 2, Semisolid Platforms first appeared in World 1-1. Four types appear: grassy (in Worlds 1, 3, and 5), sandy (in Worlds 2 and 6), snowy (in World 4), and tree-styled (in World 7).
Super Mario Bros. 3
White platforms (called White Blocks) have the special property of allowing Mario to fall into the background, behind most objects, by crouching on them for an extended time. If, as is possible in World 1-3, the goal area is reached in this state, Mario is transported to a special Toad House to receive a Magic Whistle.
Super Mario World
New Super Mario Bros. series
Semisolid Platforms appear in New Super Mario Bros., New Super Mario Bros. Wii, New Super Mario Bros. 2, and New Super Mario Bros. U. While they have new, varying designs throughout the series, the most common design in all games is that consisted of small blocks placed next to each other. Certain Semisolid Platforms in New Super Mario Bros. 2 are based off their design from Super Mario Bros. 3. Others resemble the ones in the Yoshi's Island series, but alternate between large and small segments.
Super Mario Galaxy
Super Mario Maker / Super Mario Maker for Nintendo 3DS
Semisolid Platforms appear as tools in Super Mario Maker and Super Mario Maker for Nintendo 3DS. In Create mode, they can be shaken into three different designs. In the Super Mario Bros. style, Semisolid Platforms take their default design from the otherwise solid, tree-like platforms from that game.
In the Super Mario Bros. 3 style, if Mario crouches on the white platforms for long enough, he will hop slightly, referencing the special property of the white platforms from that game, though not letting Mario drop behind the platform.
Super Mario Maker 2
Semisolid Platforms return in Super Mario Maker 2 with most of the same appearances as in the first game's four styles. In the Super Mario 3D World style, Semisolid Platforms take the form of horizontal, ladder-like platforms on top of wooden walls in the ground theme and Chain-Links in the forest theme. Cat Mario can also climb the Semisolid Platforms' walls.
Yoshi's Island series
Super Smash Bros. series
Platforms very similar in function to Semisolid Platforms known as pass-through platforms appear in many stages throughout the Super Smash Bros. series. In addition to being able to be jumped through, the player can fall through pass-through platforms by quickly tilting down on the left analog stick or tapping , depending on the control method.
Semisolid Platforms from the Mario franchise, which function exactly like pass-through platforms, also make an appearance in the Golden Plains stage in Super Smash Bros. for Nintendo 3DS, where they use the design of the solid, tree-like platforms from New Super Mario Bros. 2. They also appear in the stage Mushroom Kingdom U where they, like the stage itself, use their design from New Super Mario Bros. U.