A piece of lighting hardware from the "iron" category, most often used when hanging a conventional lighting fixture from a vertical boom. In its most basic form, consists of three parts: a lighting C-Clamp with the spud removed, a 12"-18" long, 1/2" I.D. pipe. (24" long if DTSA) and a sliding Tee fitting (or two if DTSA). A couple of abbrev.s: STSA=Single Tee Side Arm DTSA=Double Tee Side Arm City Theatrical has a line deemed Safer Sidearms. The Light Source has two alternatives: http://www.thelightsource.com/products/14/view http://www.thelightsource.com/products/31/view Any 90° pipe connector (Rota-Lock, Kee Klamp, fixed Cheeseborough, etc.) and a short length of pipe (12-36") may be used to create a sidearm. Noun or verb. See also pipe stiffener.