The jury struts on a monoplane

Monoplane

A monoplane is a fixed-wing aircraft with one main set of wing surfaces, in contrast to a biplane or triplane. Since the late 1930s it has been the most common form for a fixed wing aircraft.-Types of monoplane:...

 are small subsidiary strut

Strut

A strut is a structural component designed to resist longitudinal compression. Struts provide outwards-facing support in their lengthwise direction, which can be used to keep two other components separate, performing the opposite function of a tie...

s that join the main wing struts to the wing.

Jury struts are installed to eliminate unwanted resonance and vibration from the main wing struts and also to prevent main strut compression buckling under negative 'g'

G-force

The g-force associated with an object is its acceleration relative to free-fall. This acceleration experienced by an object is due to the vector sum of non-gravitational forces acting on an object free to move. The accelerations that are not produced by gravity are termed proper accelerations, and...

loads in the case of high-wing aircraft. They are commonly installed on braced high-wing monoplanes, but are also used on braced low-wing aircraft, where they provide extra positive 'g' capabilities.

Jury struts come in many configurations. On monoplanes with one main strut they may be just a single tube that connects that strut to the wing surface. On braced monoplanes with 'V' struts they may brace both struts to the wing surface and also provide an interconnecting tube between the two jury struts to form a jury strut assembly.