Coherent magnetic oscillations precede edge localized modes (ELMs) in TCV. The precursor has been detected prior to ELMs considered to be of type III and others previously referred to as TCV large: ELMs. This permits the identification of both as type III ELMs according to the usual classification scheme. The strong localization of these precursors on the bad curvature side of the plasma and their medium toroidal mode numbers indicate their ballooning character. Unlike conventional MHD modes, these modes start toroidally localized and grow in amplitude and toroidal extent. When the precursor encompasses the whole toroidal circumference, the increased transport phase, as indicated by the characteristic D-alpha spike, begins.