A propagation mechanism is discussed which comprises two reflections from a horizontally-stratified, partially-reflecting layer. Such a layer located appreciably below the ‘crossover height’ (i.e. the height of intersection of the horizon rays from the terminals) may cause an enhancement of signal strength which exceeds that obtained with a single reflection from a similar layer at the crossover height. This mechanism appears to give a satisfactory explanation for the experimental results obtained by Lane and Sollum in a study of layer-mode propagation over two radio paths, of length 139 and 309km, the shorter being over land and the longer partly over sea.