Abstract: A screening apparatus comprising a rectangular screening frame and a multiplicity of parallel wires which are tensioned across the screening frame has a plurality of support bars with guide slots in order to support additionally each wire. The support bars are axially rotatably and interchangeably secured to the screening frame thus allowing to use the same screening frame for different string assemblies with different distances between the wires.If guide slots for different wire spacings are provided at different sides of a support bar, the same frame and the same support bars can be used for different string assemblies.The arrangement according to the invention allows an easy and rapid change of string assemblies on the same screening frame.