Day Shapes are mast head signals indicating the navigation status of a vessel at sea to other vessels.
The meanings of the shapes are defined by the International Regulations for Preventing Collisions at Sea.
The most common signals are as follows:
Vessel Under Sail & Power: Cone (> 12m)
Anchored: Ball (> 7m (not in channel) > 20m (in anchorage))
Constrained by Draft: Cylinder
Aground: 3 Balls (vert. line) (> 12m)
Not Under Command: 2 Balls (vert. line) (> 12m)
Restricted in Ability: 1 Ball+1 Diamond+1 Ball (> 12m (except dive boats))
Fishing (commercial): 2 Cones (vert. line) (> 20m (but must display (nets or trawling) basket shape)
Vessel Towing: Diamond Tow (> 200m)
Vessel Being Towed: Diamond Tow (> 200m)