The Associated Press

Members of Congress can send letters by "franking" -- that is, putting their signature on the mail instead of postage. The cost is then charged to the members' office budgets and ultimately paid by taxpayers.

Franking -- which evolved from the Latin "francus" or free -- is older than the republic. The Continental Congress approved franking of mail sent by members beginning in 1775. The first U.S. Congress passed a franking privilege during its first session in 1789.

Franking can be used to send letters to people or small groups. A member of Congress might, for instance, use franking privileges in replying to a constituent's complaint about problems with a federal agency.

But the franking privilege also can be used to send mass mailings to thousands of people at a time. Some lawmakers send newsletters explaining their views on key issues or announcing town meetings. Some load their mailings with glossy pictures, lists of their achievements or consumer advice.

Members of Congress face some restrictions on when they can send mass mail, largely in an effort to guard against their blatant political use in campaigns.

The amount spent on mass mailings by senators is not available, but House members spent more than $20 million in 2006.