Formant frequencies vary depending on the the size and shape of the vocal tract. The first three formants (F1, F2, and F3) are the most important in speech. F1 is correlated with the height of the tongue, i.e. F1 increases as the tongue moves from high to low position. F2 is associated with the backness of the sound, i.e front sounds have higher F2 frequencies than back sounds do. F3 correlates with lip rounding; it decreases as the lips take rounding shape.