Hooked scales are particularly convenient for measuring from a shoulder.

• A. Hooked scales
• B. Sine bar
• C. Outside calipers
• D. Micrometers

Answer: (A)

Hooked scales are designed to grab the edge of a feature with a small hook, letting you anchor the measurement at a fixed reference point. When you need the distance from a shoulder to another surface, the hook catches that shoulder so the zero point sits right at the edge, while you bring the other contact into place. Reading the scale then gives you the direct distance from the shoulder to the opposite surface, which is quick and repeatable and works well even if the edge isn’t perfectly accessible. This convenience is why hooked scales are favored for measurements starting from a shoulder. Other tools work differently: a sine bar is used for angles, outside calipers need two jaws spanning a feature, and micrometers measure small thicknesses with a spindle, not conveniently anchored to a shoulder edge.