WASHINGTON — Eager to encourage letter writing, the post office is trying a new tack, offering to let people mail a card to a friend for free.

Until March 31, people can get a free card, postage included, which they can use to send a message to a friend.

The cost of the project was not immediately available, officials said Thursday. Covering the cost is Home Box Office. The cable network is promoting its miniseries on John Adams, much of which is based on the letters of the second American president and his wife, Abigail.

The free cards are available at www.poweroftheletter.com by clicking on Free John Adams Greeting Card.

"Letters and cards are personal, you can hold them, you can read them over and over again, and keep them forever — these are things that e-mail and text messaging cannot replace," said Anita Bizzotto, chief marketing officer for the Postal Service.

It is one to a customer, the Web site notes.

Long the largest proportion of mail, first class letters have fallen behind advertising mail in volume as people turn to telephone and the Internet for messaging.

So the post office is hoping to get people to pick up a pen to write a message that can be saved for years.

John and Abigail Adams exchanged over 1,100 letters during the course of their courtship and marriage.

The letters, held by the Massachusetts Historical Society, were a crucial resource for historian David McCullough, whose biography of John Adams is the foundation for the HBO miniseries.