Vocational interest inventories are often designed, scored and interpreted in accordance with the six Holland types (R, I, A, S, E and C) and their hexagonal arrangement. A three-letter code then identifies the three types on which the person scored most highly. The congruence between a person and their environment (e.g. workplace or field of study) may affect their satisfaction and performance, and how to define congruence is a question of long standing. The method proposed here is based on (a) the presence and position of each of the six Holland types in the two three-letter codes being compared (or their absence from the codes) and (b) proximities of all pairs of types, reflecting the hexagonal arrangement of the types. There is discussion of whether the hexagonal arrangement ought to be used in calculating congruence, but a definite answer is not yet clear.