Different crystal cuts are used depending on operating frequency and application, but most of today’s resonators primarily use the AT-cut. This illustration shows how an AT-cut plate and a BT-cut plate are related to the directions in a quartz bar. The illustration shows how the AT-cut’s Frequency vs. Temperature characteristics change as the q angle of cut is varied in one minute of arc increments. The Dq = 0 curve shows the Frequency vs. Temperature characteristic at the “reference angle”. At the reference angle, the turnover points coincide with the inflection point. The reference angle varies with design, i.e., with overtone, plate contour, etc.