What this suffix does
-metry comes from Greek "metron" (measure) and forms nouns that describe the science, system, or practice of measuring something. Geometry (measuring the earth), trigonometry (measuring triangles), symmetry (measuring together, equal measurement), photometry (measuring light), biometry (measuring living things) — each describes a systematic approach to measurement in a specific domain. While the related suffix -meter describes the instrument or unit of measurement, -metry describes the entire science or practice of measuring in that domain.