Flanking

Rules As Written

Can use an action to "Aid Another" as described in PHB.

Optional rule, listed on p. 251 of the Dungeon Master's Guide, grants advantage on all attacks where an ally is on the opposite side. DMG has pictures to illustrate.

House Rule

Flanking an enemy (being on opposite sides of the token from your ally) allows you to give your ally advantage on their attacks against the foe as a bonus action.

Rationalization/Reason for the Rule

Encourages tactical movement without making combat devolve into a conga line.