The origin of the first sharp diffraction peak (FSDP) in the total structure factor of disordered network-forming glasses and liquids is frequently interpreted either as resulting from crystalline-like layers or from correlations between basic structural units (clusters) and interstitial voids. By applying appropriate criteria to configurations of liquid SiO2 and GeSe2 obtained by first-principles molecular dynamics, we find that neither interpretation holds. Despite the appearance of a FSDP in both liquids, there is no evidence for the occurrence of crystalline-like layers. By contrast, cluster-void correlations do not only occur for these liquids, but also for a model system in which the FSDP is absent. (C) 2001 American Institute of Physics.