Link

Link is a theoretical static ability which was discussed by R&D, before it was redesigned as meld. Theoretically, link cards would be double-faced cards with the link ability in each face. Each face would represent a half of a spell a left side or a right side. Thus, it would require to have two link cards in your hand in order to cast them as a link spell, just pairing a left side of one card with the right side of the other.

History
Link was created for New Phyrexia by Ken Nagle. At that time it was referred to as 'Mechanic X'. Nagle was inspired by the card from Unglued. The idea was that you could mix and match to create a variety of creatures, and that each half would define part of what the creature was, both how big and with what abilities. But there were both card-layout, rules and templating concerns. Making the link cards work would require a lot of time and attention, so Aaron Forsythe cut this mechanic from the set during devign.

Despite its failure, R&D still hoped to use the mechanic in a future set. It was finally released as meld in Eldritch Moon. It also inspired the Augment cards from Unstable.