Question: Why is the president's wife called the First Lady?
Answer: James Buchanan assumed the American presidency in 1857
and became the first (and only) bachelor president. Since
Buchanan did not have a wife, he turned to his charming niece
Harriet Lane to take over the social functions at the White
House. During her uncle's presidential tenure, Ms. Lane became
Washington's most popular hostess and she used her special
position to promote certain social causes. Consequently, people
began to refer to her as the "First Lady" since she was the
president's niece---not his wife---and thus had no formal title.
Somehow the title stuck.