Chart No. 1 · Q 21
Can buoy (cylindrical)
Buoys & beacons
A square-ish glyph is a can buoy — the flat-topped shape used for port-hand marks in IALA B waters.
What it means
A rectangular glyph on a position circle is a can (cylindrical) buoy — flat-topped in real life.
Why it matters
In US (IALA B) waters cans are green port-hand marks ('leave green cans to port returning'); in Region A cans are red port-hand marks. The charted shape plus the color letter tells you which side to pass.