The Fillet or Radius Gage is an external and internal radius on each leaf that permits both concave and convex surfaces to be measured. The leaves are specially shaped for use in any position at any angle to measure fillets and radii in corners or against shoulders.

The Starrett 279 has twenty leaves with radii from .020-.400" inclusive. Nine leaves have concave and convex radii from .020-.10" in increments of .010", four leaves with concave and convex radii from .125-.20" in increments of .025", one leaf with concave and convex radii of .250", three leaves with concave radii only from .300-.400" in increments of .050" and three leaves with convex radii from .300-.400" by an increment of .050".