What a diamond day shape tells you
A diamond is a daytime shape, not a light. Rule 24 requires a diamond when the length of a tow exceeds 200 meters, measured from the stern of the towing vessel to the after end of the tow. The same diamond can also appear on the towed object itself when that object is displaying its own towing shapes. Rule 27 uses the diamond differently: a vessel restricted in her ability to maneuver shows three shapes in a vertical line, with balls as the upper and lower shapes and the diamond in the middle. A dredge or underwater-operations vessel can also show two diamonds on the side where another vessel may pass.
Exam traps
Do not read every diamond as "restricted in ability to maneuver." A single diamond is the long-tow clue. Ball-diamond-ball is the RAM clue. Two diamonds to one side of a dredge indicate the safe passing side. Watch the wording around tow length: the rule says "exceeds 200 meters," not "equals 200 meters." At night, the question will usually switch from shapes to the matching towing lights or red-white-red RAM light pattern.
Related day-shape clues
Pair this with the cylinder shape for constrained-by-draft vessels and the two-ball display for vessels not under command. The exam often tests day shapes as a group because the answer turns on the combination, not the individual geometric symbol.