What's the difference between a Direct-reading gage and a Comparison-type/Comparative gage?

Direct-reading gages are typically meant to be calibrated once at any point within the measuring range. The gage can then measure the exact dimensional value anywhere within the range. Gages such as micrometers and calipers are considered direct-reading gages.

Comparison-type gages are typically used to measure only the displacement from the nominal value. These are "zeroset" or preset at a specificn nominal value which is typically the master part, but can also include setting rings or gage blocks at the Nominal value. Comparison-type gages usually have a limited measuring range. Gages such as dial bore gages (not including Borematics and Holtests) and snap gages are considered comparison-type.