Linked

A monster is in a Linked State (リンク, Rinku Jōtai) if its Link Marker is pointing to a monster, or if it has at least one Link Monster's Link Marker pointing to it.

Mutual Link
A Mutual Link (リンク, Sougo Rinku) is a kind of Link State that occurs when two Link Monsters' Link Markers are pointing to each other, such as if a Link Monster with an Up Link Marker is below a Link Monster with a Down Link Marker.

Extra Link
An Extra Link enables a player to occupy both Extra Monster Zones simultaneously. A player can perform an Extra Link, provided that:
 * 1) The player controls a Link Monster in an Extra Monster Zone and the other Extra Monster Zone is unoccupied.
 * 2) The three middle Main Monster Zones on one side of the field must be occupied by Link Monsters.
 * 3) Each of the Link Monsters involved in the Extra Link must be in a Mutual Link with the orthogonally adjacent Link Monsters that are also involved in the Extra Link.
 * 4) The Link Monster involved in the Extra Link occupying the Main Monster Zone in the same column as the unoccupied Extra Monster Zone must have a Link Marker pointing to that unoccupied Zone.
 * 5) The Link Monster to be Summoned to the unoccupied Extra Monster Zone would be in a Mutual Link with the monster involved in the Extra Link that is in the same column as it, and Summoning it would not disturb the setup of the Extra Link (e.g. by using the monsters involved in the Extra Link as Link Materials).

If all of the above conditions are fulfilled, the player can Special Summon a monster to the unoccupied Extra Monster Zone, thus performing an Extra Link.