In music it is called counterpoint, whereas in painting, counterchange and is allied to light and shade. It is the effect of changing a dull ridge in the middle distance into a more interesting feature, without making it compete for light and space or indeed the centre of interest or focal point. This is achieved by painting one side dark against a light sky, and the other side light against a dark sky. Gradually, changing the tones works best in these instances. Other examples are fences, hedgerows, bridges or walls, partly in sunlight, or by basic tonality in the immediate foreground. On the coast see how seagulls change from white to dark due to internalised shadows, against dark cliffs or the light sky. You do not even need a sketch book but observe and soak it up.