S is the stress on the crystal. "b" is defined as the piezoelectric stress coefficient. F and E are both functions of the applied force. Thus, as a piezoelectric crystal is exposed to an electric field, you can define the applied pressure and stress due to the field presence. The larger the piezoelectric coefficient, the more piezoelectric character a compound/material has. Some examples of compounds with large piezoelectric coefficients are ammonium dihydrogen phosphate, potassium dihydrogen phosphate, ethylene dihydrogen tartrate, and alpha-quartz. The quartz, being the most commonly used piezoelectric material, has the smallest coefficients on the list. Resonators are the dominant application for piezoelectric materials. As described earlier, when a piezoelectric material is exposed to an electric field, the crystal distorts with the direction of the field. One can cause the crystal to resonate by using a switching electric field. A very simple example of this would be a crystal in the electric field caused by a wire carrying AC current. As the field changes with the current, the crystal will stretch and compress (i.e. resonate) with the field (see Figure 1). The frequency of the resonation depends on many factors. First, is the strength of the applied field. The stronger the field, the more distorted the crystal will become in response. The cut of the crystal also is a factor in frequency. The dimensions of the cut (length, width, thickness, total mass) play a major role in how much the crystal bends/stretches in the field. Another less obvious factor in how the cut of the crystal effects the frequency of vibration is the orientation of the cut from the bulk crystal. One can make a cut out of a bulk crystal on the x, y, or z-axes. The difference between each cut is how the resonation propagates through the crystal. A x-plane cut crystal exhibits longitudinal strain when put under an electric field.