A comparator is a precision metrology instrument that does not read a dimension on its own scale from zero; instead it measures the small deviation of a workpiece from a known standard. It therefore works on the principle of relative (comparative) measurement.

The typical procedure is:

• The instrument is first set (zeroed) against a reference of known size — a slip (gauge) block or master gauge equal to the nominal dimension.
• The workpiece is then substituted, and the pointer or display shows only the difference (say +0.01 mm or −0.02 mm) from that master.
• This differential principle allows a small motion to be greatly magnified (mechanically, optically, pneumatically or electrically), giving very high sensitivity and repeatability.

Because it always needs a reference standard, a comparator cannot give the absolute size directly. This distinguishes it from an absolute or direct-reading instrument such as a micrometer or vernier caliper, which carries its own graduated scale and yields the full dimension without an external master. "Basic measurement" is not a defined metrology category for this instrument. Hence a comparator performs relative measurement.