A few biplanes like the Stearman have a strut parallel to the fuselage, halfway along the rigging's landing wires and flying wires, not fastened to the airframe, not bearing any obvious load. One on each side. What are they for? What are they called?
I'd guess that because some of the Stearman's wires are doubled (also unusual?), the strut prevents them from banging into each other and chafing. But that seems unlikely because their tension is 800 to 1400 pounds, according to page 24 of the Dec. 2007 issue of Vintage Airplane (p. 26 of the pdf). Even if a maneuver temporarily halved that tension, deflecting one of those by two inches to hit its twin would take tons of force.
(image source: Wikimedia)