Blue parallel lines cut across by a red transversal
The diagram shows a pair of allied angles (p and v). In some textbooks these are called interior angles. Move points B and C to make v take a range of values and see how p is affected. For example, make v = 100º, 110º, 120º, 130º and 140º and note the size of p in each case. There are two pairs of allied angles - click the check box to look at the other pair (q and w). Move points B and C to a variety of positions and note how q and w relate to each other. So what have you deduced about allied angles?