Lady Bird Johnson may have said it best: "The first lady is an unpaid public servant elected by one person — her husband."

Ever since there have been presidents, there have been presidential spouses who have played important, but evolving, roles in both government and history.

"They have always played a strong role," says Pat Krider, executive director of the National First Ladies' Library in Canton, Ohio. A lot of people look to Eleanor Roosevelt as the role model for the modern first lady, but it didn't start with her, says Krider. "It goes way back. Each first lady finds a way to contribute. Some were in the background, some were out front. But they each brought something unique to the job."

The library was established in 1997 by Mary Regula, who felt there should be more information and education out there on the first ladies and the impact they've had. In 2000, Congress created the First Ladies National Historic Site, which is administered by the library, in the historic Canton home of first lady Ida Saxon McKinley.

They weren't always called first ladies, of course. That term did not come into common usage until after 1877, when it was used in newspapers to describe Lucy Hayes as she accompanied her husband on a trip from Washington to California.

In the early days of the republic, various names were used for the president's wife. Martha Washington was often called "Lady Washington," but other titles known to have been used were Mrs. President and Mrs. Presidentress.

Legend has it that Zachary Taylor referred to Dolley Madison as "first lady" in a eulogy at her funeral, but no written record of the speech exists. The name was used in Washington social circles by the mid-1850s. An 1863 diary entry of newspaper reporter William Howard Russell mentioned "gossip about the first 'Lady in the Land."'

Although the first lady has no specific duties under the Constitution, it became clear early on that an official hostess to preside over the White House and at official functions was necessary.

Thomas Jefferson was a widower of 18 years when he came to the White House. His daughter, Martha, frequently served as his official hostess. Over the years, more than a dozen other women — most often daughters, but sometimes other relatives and friends — filled the role for presidents whose wives had died, were ill or unable to attend to duties.

Those women are part of the National First Ladies' Library — as are the presidential wives who died before their husbands took office.