Young couples have long signaled their devotion to each other by various means — the gift of a letterman jacket, or an exchange of class rings or ID bracelets. Best friends share locker combinations.

The digital era has given rise to a more intimate custom. It has become fashionable for young people to express their affection for each other by sharing their passwords to e-mail, Facebook and other accounts. Boyfriends and girlfriends sometimes even create identical passwords, and let each other read their private e-mails and texts.

They say they know such digital entanglements are risky, because a souring relationship can lead to people using online secrets against each other. But that, they say, is part of what makes the symbolism of the shared password so powerful.

“It’s a sign of trust,” Tiffany Carandang, a high school senior in San Francisco, said of the decision she and her boyfriend made several months ago to share passwords for e-mail and Facebook. “I have nothing to hide from him, and he has nothing to hide from me.”