Royal Ancestry & Royal Descent
A Royal Descent is a descent (often through female or illegitimate lines) from one or more Kings or Queens.
Royal descent is sometimes claimed as a mark of distinction. By researching your family history you can see if you have royal blood in your veins!
It is a long tradition that royalty marry only those of their own class. Because of this, "the ruling houses of Europe have always been closely related to one another", and the descent from one monarch will be found in many other families - all present European monarchs are descendants of William I of England, for example.
Below is a the first page of the 'Royal Pedigree' from 'Burke's Peerage and Baronetage 1921', and shows a detailed family tree of the monarchs of England. The image is from a PDF document of the scanned book, from S&N Genealogy Supplies, and there is also 'Burkes Peerage and Baronetage 1885' available.
Click for full size image