The six parts of a triangle may be written in cyclic order as ("aCbAcB"). The cotangent, or four-part, formulae relate two sides and two angles forming four "consecutive" parts around the triangle, for example ("aCbA") or ("BaCb"). In such a set there are inner and outer parts: for example in the set ("BaCb") the inner angle is "C", the inner side is "a", the outer angle is "B", the outer side is "b". The cotangent rule may be written as (Todhunter, Art.44)