The best way is to follow this link:

http://en.wikipedia.org/wiki/Line_of...British_Throne

This is all vested in tradition and history. It must be seen in light of events of the time.

In essence, it is eldest sons, their sons; other sons, their sons; daughters, their sons. When that runs out then the line goes sideways from the currant monarch, to include cousins and nephews. Of course, illegitimacy or being a Roman Catholic (or married to a Roman Catholic) rules one out of the succession.

When Catherine produces a child, the whole 2000 odd all shuffle down to make room.

Regards

Chas