Two transport systems evolved in the early video machines, known as the alpha wrap and the omega wrap. In the alpha-wrap machines the tape is wrapped around the head drum for a full 360 degrees (the tape looking like the lowercase Greek letter alpha). There is only one head which writes a complete stripe for every revolution of the head. This system has problems when the head transits from one piece of tape to the next, giving a large signal gap between fields. The machine has to fill this gap with the frame-synchronizing pulses. Such machines are constrained to using guard-band recording (see below).