Historical Rules · 16 July 2010
Rule 303.4
Official source text from the 16 July 2010 Rules release
Some enchantments have the subtype "Aura." An Aura enters the battlefield attached to an object or player. What an Aura can be attached to is restricted by its enchant keyword ability (see rule 702.5, "Enchant"). Other effects can limit what a permanent can be enchanted by.