亲戚称谓英语，first cousin once removed

亲戚称谓英语，first cousin once removed
求高人指点。这个问题很困扰我。就是英语里面这个家族谱系的说法与算法。cousin有first cousin,second cousin... ，还有后面连接once removed, three times removed,等等。请问是啥意思。比如下面这段话。
Franklin Roosevelt: fourth cousin three times removed of John Quincy Adams, third cousin four times removed of Martin Van Buren,
half fourth cousin three times removed of Zachary Taylor, seventh cousin once removed of Millard Fillmore,
sixth cousin twice removed of Franklin Pierce, fourth cousin once removed of Ulysses Grant, sixth cousin once removed of Rutherford Hayes,
half eighth cousin of Grover Cleveland, fifth cousin of Theodore Roosevelt (although his wife, Eleanor Roosevelt, was a niece of Theodore),
sixth cousin twice removed of William Taft, seventh cousin once removed of Calvin Coolidge.
请高手不吝赐教。请高手不要仅仅翻译原文，最好能给小弟说说在这个系谱学上，是啥意思。比如Once removed隔一代
还有这句话，But in fact, Mr. Obama and Mr. Limbaugh are also tenth cousins – albeit, once removed.
好的话，有加分！