Experiments were conducted on a vacuum gap with a trigger electrode mounted in one of the main electrodes. Waveforms under various circuit conditions, photographic and visual observations, probe measurements, and various trigger materials were studied to investigate the triggering mechanisms. Different mechanisms are proposed according to the polarity of the remote main electrode. For positive polarity, the trigger arc establishes its own cathode spots, which then supply current to the main discharge, dividing to form new spots as needed. For negative polarity, the plasma produced by the trigger produces a concentration of the main electrode potential difference across a thin sheath adjacent to the surface of the remote main electrode.