Historical Rules · 25 August 2017
Rule 701.3a
Official source text from the 25 August 2017 Rules release
To attach an Aura, Equipment, or Fortification to an object means to take it from where it currently is and put it onto that object. If something is attached to a permanent on the battlefield, it’s customary to place it so that it’s physically touching the permanent. An Aura, Equipment, or Fortification can’t be attached to an object it couldn’t enchant, equip, or fortify, respectively.