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Girders are construction platforms found in various games, starting with the original Donkey Kong. Most of their appearances since then have been in reference to this appearance.
Donkey Kong / Donkey Kong (Game Boy)
Girders first appear in Donkey Kong, appearing in every stage, most notably in 25m, where they are the only type of platform. In the Game Boy remake, they appear in all of these stages, as well as several stages in the game's later worlds, such as Big-City. In both games, they act essentially as a semi-solid platform that Mario can walk on.
Mario vs. Donkey Kong series
Generic girders appear in every Mario vs. Donkey Kong game. Throughout the series, they act as semi-solid platforms much like those in Donkey Kong, being red-colored in Mario vs. Donkey Kong, Mario vs. Donkey Kong 2: March of the Minis and Mario vs. Donkey Kong: Minis March Again!, and gray with rounded corners in all of the following titles. The latter are described as "gray steel frames".
This type of fixed girder is distinct from one of the series' staple gameplay mechanics, the Red Girders, which first appear in Mario vs. Donkey Kong: Mini-Land Mayhem! These can be drawn between red rivet blocks using the touchscreen to help the Minis cross gaps or reach a higher area.
In Mario vs. Donkey Kong: Minis March Again!, pieces of red-colored girders are among the objects that fall from the top of the screen when Donkey Kong slams his fists in Floor 1 Donkey Kong. Short girders appear first, followed by longer ones that are harder to avoid after DK is hit a third time. The player loses a Mini toy if a girder crashes into it.
Super Smash Bros. series
Donkey Kong Country Returns
In Donkey Kong Country Returns, girders are the primary floor in the Factory world. In the background for the Factory's first level, Foggy Fumes, girders arranged in the zig-zagged 25m layout can be seen at one point. This pattern also commonly appears on wall reliefs throughout the temple-themed levels, along with other depictions of Donkey Kong arcade graphics.
Super Mario Odyssey
Girders appear in Super Mario Odyssey, where they are seen as common platforms throughout the Metro Kingdom. They do not act as semi-solid platforms in this game, but items such as Coins and Power Moons can be found inside some of them.
Other appearances and allusions
In the manual for Donkey Kong 64, Cranky Kong hopes that a level design he submitted, "Girder Grapple," made it into the game as a secret level, referencing him being the original Donkey Kong from the arcade game. While this level doesn't appear, the original game can be played in an arcade cabinet found in Frantic Factory.