Dimensional inspection chooses a measurement method whose geometry and resolution match the feature being checked.

A caliper measures distances between contacting jaws and is versatile for external, internal and depth dimensions. A micrometer uses a precision screw to convert rotation into small axial motion and is usually more specialized but can provide finer resolution and a more controlled measurement geometry.

A comparator measures deviation from a reference rather than determining absolute size directly. This is useful when many parts must be checked relative to a nominal standard.

Measurement force and alignment matter. If a micrometer is tilted across a cylindrical diameter, the contact geometry no longer represents the intended diameter. Excessive force can also deform soft or thin parts.

Instrument resolution must be meaningfully smaller than the tolerance being judged. If a dimension is allowed to vary by only $\pm0.01$ mm, an instrument that reports only to $0.1$ mm cannot discriminate acceptable from unacceptable parts.

For repeated production, inspection often becomes a comparison problem:

reference / calibration
        ↓
measure feature consistently
        ↓
compare result with specification

Dimensional metrology is therefore not 'reading a number from a tool'. It requires selecting contact geometry, measurement range, resolution and procedure that correspond to the drawing requirement.