A way of setting the edit timebase to a figure that is different to the fps. For example when we edit in 50p there are 50 frames per second. When this is converted to a 25i clip for broadcast each frame effectively becomes a field of the interlaced clip. Broadcast regulations stipulate that all cuts must be on the upper or odd field, which means that all edits need to be on even frames. Avid allow a way of ensuring this by creating a 50P project but setting the timebase to 25p ensuring cuts can only occur every second frame, meaning that when converted to interlaced it will conform with broadcast requirements.