Glasgow, like London, was divided into compass districts: C, W, NW, N, E, SE, S, SW. When postcodes were introduced, these were mapped into the new "G" postcode: C1 became G1, W1 became G11, N1 became G21, E1 became G31, S1 became G41, SW1 became G51, and so on. As NW and SE had never been subdivided they became G20 and G40 respectively.