Under the Kaplan/Brightwood section on rigid frames in lesson Ten: Long Span Structural (using the 4.0 materials):

"Rectangular frames are most often two-hinged. While gabled frames may be two - or three - hinged."

Wait, what? I thought by definition rigid frames were not hinged, and this is what gives them continuity and allows them to transfer their load.

This is referring to equations of equilibrium. For example, a two-hinged arch is statically indeterminate but a three-hinged arch is determinate.