Abstract: Fibrous substrates intended for the production of composite bodies are produced from sheet material consisting of fibrous filaments or threads bound into a coherent sheet with a binder by providing fine, substantially uniformly sized and distributed perforations in at least selected areas of the coherent sheet, subdividing the perforated sheet into a plurality of substantially identical smaller flat sheets each comprising at least one of said selected areas, stacking the smaller sheets one upon another so that in the resulting stack the perforations in one sheet overlie the perforations in the immediately underlying sheet and thus form passages through the stack and thereafter threading through said passages in said stack strands or rods formed from fibrous filaments or threads which have been bound together with a binder.