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Here's my understanding of grip terminology. Knuckle, aka grip, refers to the assembly with two disks, central screw post, and threaded handle. Handle turns to squeeze the disks together like a vise. You insert an arm to gain distance, a pin which is part of a small frame, an "ear" which is part of a large frame, or even a baby pin to attach a light. Since a bare knuckle isn't very useful, it is usually attached to something: baby receiver (for grip head), arm (grip arm aka gobo arm), a second knuckle (double gag), second knuckle sans handle (gag). Stands are even built with (large) knuckles attached for overhead frames (butterflies), and scrims for hot lights.