  We present a theory for pitch, a matrix property which is linked to the
coupling of rotational and translational motion of rigid bodies at low Reynolds
number. The pitch matrix is a geometric property of objects in contact with a
surrounding fluid, and it can be decomposed into three principal axes of pitch
and their associated \textit{moments of pitch}. The moments of pitch predict
the translational motion in a direction parallel to each pitch axis when the
object is rotated around that axis, and can be used to explain translational
drift, particularly for rotating helices. We also provide a symmetrized
boundary element model for blocks of the resistance tensor, allowing
calculation of the pitch matrix for arbitrary rigid bodies. We analyze a range
of chiral objects, including chiral molecules and helices. Chiral objects with
a $C_n$ symmetry axis with $n > 2$ show additional symmetries in their pitch
matrices. We also show that some achiral objects have non-vanishing pitch
matrices, and use this result to explain recent observations of achiral
microswimmers. We also discuss the small, but non-zero pitch of Lord Kelvin's
isotropic helicoid.
