In sailing, the quarter refers to the rear sides of a vessel, located between the widest part (beam) and the rear (stern), spanning the port and starboard aft-quarters. It designates the rearward quadrant on both sides of the boat, crucial for defining relative positions, such as spotting vessels or managing wind.

Key Aspects of the Quarter:
Location: The port quarter is the aft-left side, and the starboard quarter is the aft-right side.

Reference Point: It is roughly a 45-degree bearing from the center of the stern (dead astern).

Usage: Used to describe where wind is coming from ("a wind on the quarter") or to locate objects, such as a buoy or another boat approaching from "over the quarter".

Contrast: It is the opposite of the bow (front) of the vessel.

Quartering Sea: Refers to waves approaching the vessel from this specific rear-quarter angle.