Facing

Source: Dungeon Master’s Guide p. 252

If you want the precision of knowing which way a creature is facing, consider using this optional rule. Whenever a creature ends its move, it can change its facing. Each creature has a front arc (the direction it faces), left and right side arcs, and a rear arc. A creature can also change its facing as a reaction when any other creature moves.

A creature can normally target only creatures in its front or side arcs. It can’t see into its rear arc. This means an attacker in the creature’s rear arc makes attack rolls against it with advantage.

Shields apply their bonus to AC only against attacks from the front arc or the same side arc as the shield. For example, a fighter with a shield on the left arm can use it only against attacks from the front and left arcs.

Feel free to determine that not all creatures have every type of arc. For example, an amorphous ochre jelly could treat all of its arcs as front ones, while a hydra might have three front arcs and one rear one. On squares, you pick one side of a creature’s space as the direction it is facing. Draw a diagonal line outward from each corner of this side to determine the squares in its front arc. The opposite side of the space determines its rear arc in the same way. The remaining spaces to either side of the creature form its side arcs.

On hexes, determining the front, rear, and side arcs requires more judgment. Pick one side of the creature’s space and create a wedge shape expanding out from there for the front arc, and another on the opposite side of the creature for the rear arc. The remaining spaces to either side of the creature are its side arcs.

A square or hex might be in more than one arc, depending on how you draw the lines from a creature’s space. If more than half of a square or hex lies in one arc, it is in that arc. If it is split exactly down the middle, use this rule: if half of it lies in the front arc, it’s in that arc. If half of it is in a side arc and the rear arc, it’s in the side arc.