Sit makes it possible to use slabs and stairs like chairs. Simply rightclick a right side up slab or stair block with an empty hand to sit on it. To exit, press the key you use to exit a minecart (by default SHIFT). Usually, slabs/stairs from other mods are supported, however if any do not work, please contact me.
This mod works on singleplayer as well as multiplayer, but needs to be installed on both sides if it's used on a server.
You are allowed to use this mod in any modpack, it would be cool if you'd let me know if you do so.
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