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A beam is used for support. For example, you can use beams to support a floor made of planks on the 2nd floor of a building.
To use beams, you must have a Brick Plug, Log, or Wooden Column to attach it to. Beams can go 2 out from a Log, Brick Plug, or Wooden Column, but can be 5 long with a Brick Plug, Log, or Wooden Column on both ends of it. To extend the length of a beam further, check the wiki page on Braces.