This note looks into some of the core ideas of the well-known combining formula of Bates-Granger-Newbold-Reid and also the corresponding sign-determination rule of the optimal combining weights under the error-variance minimizing criterion by means of a set of diagrams. These geometric constructions, instructive in itself, highlight intuition behind the algebraic formula. In particular, the optimal location in the weight space is demonstrated to occur at a point where a surface of an ellipsoid w'Σw = r2 is tangent to the hyperplane w'v = 1. The geometric illustrations can also depict the possibilities of the combination of two positive weights as wel1 as a positive and a negative weight. (JEL:C53)