Explanation of the deduction of the offset between the hypothetical position of a H II region formed in the centre of the arm and its real position for a clockwise-rotating galaxy. The trailing spiral arm is represented by the thick line. The hypothetical H II region is represented by the open circle. The real position of the H II region is indicated by the filled circle. The angles θ_i, θ_R, θ_A, and the offset δθ (between the arm and the theoretical position of the H II region) are indicated by an arc and the appropriated label. In this diagram it is assumed that the H II region is inside the co-rotation radius of the galaxy.