The upper rectangle shows the forward proposal window, while the lower rectangle shows the reverse proposal window. Dragging the mouse above the axis changes the value of θ (the current parameter value). Dragging the mouse below the axis changes the value of θ* (the proposed parameter value). If θ* lies in the dark purple zone, then that value could have arisen either because it was proposed directly or it was reflected to that spot after a point to the left of zero was proposed. Note that if the forward proposal results in a value of θ* in the dark purple zone, then the reverse proposal results in a value of θ in a dark purple zone, and if the forward proposal results in a value of θ* in the lavender zone, then the reverse proposal results in a value of θ in the lavender zone. Thus, the proposal is symmetric and the Hastings ratio equals 1.