Hooked scales are especially convenient when measuring from a shoulder.

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

Answer: (A)

Hooked scales are especially convenient when measuring from a shoulder because the hook on the end can grab the edge of the shoulder, giving a definite starting reference. With the zero set at that edge, you can slide the scale along the surface to the opposite face while the hook holds the shoulder in place. This creates a stable, repeatable contact point and prevents the tool from slipping, so the reading reflects the distance from the shoulder to the other surface. This is especially useful for features where a flat rest isn’t possible, like measuring from a step or shoulder to another surface. Other tools don’t anchor to an edge as reliably: a micrometer isn’t designed to start from a shoulder, and instruments like a sine bar or outside calipers don’t provide the same edge-contact stability.