The normalization factor in the star-triangle relation can be evaluated in a simple form by taking determinants. If we combine this with the rotation symmetries, then we can show that a certain simple quantity I has to be independent of the rapidities. In this sense it is an invariant. We evaluate it for several particular models and find it is one for self-dual models, and is related to the modulus k (or k ′ ) for the Ising, Kashiwara-Miwa and chiral Potts models. It may be possible to absorb S(a) into W pq (a, b) and/or W pq (a, b), but to the author it seems clearer not to do so. † There are other solvable planar models, derived from the three-dimensional Zamolodchikov model, which are asymmetric, but these satisfy the "star-star" relation, rather than the star-triangle. [2]