It is shown in this note how, by a simple geometrical construction, the two fundamental solutions of the equation a cos θ + b sin θ = c (to which ± 2 k π added gives, if required, the general solution) may be determined experimentally, using the protractor, without ambiguity of any kind. The method does not involve any laborious graph-tracing, and is at once simpler and more accurate than the geometrical discussions given in one or two text-books. (See, for example, Loney, Part I., page 141, §130; Levett & Davison, page 138).