Blue parallel lines cut across by a red transversal
The diagram shows a pair of alternate angles (p and w). These are sometimes called Z angles but alternate is their proper name. Move points B and C to a variety of positions and note how p and w relate to each other. There are two pairs of alternate angles - click the check box to look at the other pair (q and v). Move points B and C to a variety of positions and note how q and v relate to each other. So what have you deduced about alternate angles?