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This allows you to grow, shrink, "lower" or "raise", and rotate the object in a variety of ways. Furniture can be upside down in wreckage, the floor can have flags on it, the walls can have carpets. Many objects are not optimised to be rotated however!
Shortcuts
Holding ALT and holding the mouse over the placeable will move it up and down.
Holding CTRL and moving the mouse wheel will scale them.
Map Bounds
Did you know? There is a way you can get placeables to be outside the map bounds and still load in the game.
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