  Tilt-to-length coupling is a technical term for the cross-coupling of angular
or lateral jitter into an interferometric phase signal. It is an important
noise source in precision interferometers and originates either from changes in
the optical path lengths or from wavefront and clipping effects. Within this
paper, we focus on geometric TTL coupling and categorize it into a number of
different mechanisms for which we give analytic expressions. We then show that
this geometric description is not always sufficient to predict the TTL coupling
noise within an interferometer. We, therefore, discuss how understanding the
geometric effects allows TTL noise reduction already by smart design choices.
Additionally, they can be used to counteract the total measured TTL noise in a
system. The presented content applies to a large variety of precision
interferometers, including space gravitational wave detectors like LISA.
