Think the CD Is Over? Not So Fast : Artists are including uncredited tracks on their discs. It’s a concept of the ‘90s, but the idea’s not new.

Don’t take that CD out of the player just yet.

You may think you heard the last track--at least that’s what it says on the album jacket. But there’s a good chance there’s more.

Many artists are adding uncredited tracks to the CD versions of their albums--from Alanis Morissette and Pearl Jam to Janet Jackson and R.E.M.

These so-called hidden tracks offer a bonus to fans while giving artists an opportunity to add songs that might not fit sonically or thematically with the rest of the album. Sometimes these extra cuts don’t even start playing until several minutes after the last credited song has faded out, often startling the unsuspecting listener who’s left the CD in the player.

“It’s like a little coda, a little end piece,” says Tim Devine, vice president of A&R; at Capitol Records. “If there is a track that is so different from the rest of the record that you want to set it apart, this is a good way to go.

“For years, people have been putting out unreleased album tracks as B sides of singles. This is just a way of including an extra track in the package to give the public something more than it bargained for.”

The addition of hidden tracks has proliferated in recent years because the CD format, which allows for up to 80 minutes of material on each disc, provides that much more space for adventurous artists to leave their marks.

Morissette, whose biting raw-edged alterna-rock songs earned her six Grammy nominations, added the a cappella “Your House” to the end of her best-selling album “Jagged Little Pill.”

“The record was essentially finished,” she says, “and [producer and co-writer] Glen Ballard and I were having a hard time letting go of the writing of it, so we wrote another song that was more of an addendum, an epilogue of sorts.”

A few hidden tracks have become hits or received extensive radio airplay, including “Eurotrash Girl” from Cracker’s album “Kerosene Hat” and “Whoops” from Janet Jackson’s “janet.”

KSCA-FM (101.9) has championed several hidden tracks, including “Horse” from Live’s album “Throwing Copper” and “Possession” from Sarah McLachlan’s “Fumbling Toward Ecstasy.” The McLachlan track is an alternate acoustic version of the hit song that kicks off the album.

“We love hidden tracks,” says Merilee Kelly, music director at KSCA. “We’re just like the fans: We open up the CD and see what’s there. I talk to record company representatives who don’t even know there’s a hidden track on an album, so nobody’s pushing these tracks on us.”

Willie Nelson tried something new on his album of standards, “Moonlight Becomes You,” adding a hidden track before the first song. Essentially a spoken explanation from Nelson about why he chose the songs included on the album, the track is accessible only by rewinding past the start of the disc.

Diamond Rio’s upcoming “IV” album also includes a hidden track before the opening cut.

“I wouldn’t call it a song,” says Mike Clute, co-producer of the album. “It’s a piece that we created for the opening of their show. It’s an intro piece that consists of bits and samples and licks and things from some of their hits.

“It’s something that’s really cool to listen to, but it’s not a mandatory part of the album. It was just something fun to do.”

That’s the key to hidden tracks, says Pete Howard, editor and publisher of the CD magazine ICE.

“There’s no hidden meaning,” he says. “It’s just a way of having fun with the CD format.”

Uncredited tracks, however, actually predate the CD era.

The Beatles added unfathomable messages in the run-out groove of “Sgt. Pepper’s Lonely Hearts Club Band” and tacked on the uncredited “Her Majesty” to the end of “Abbey Road.”

And the Clash’s first U.S. hit, “Train in Vain,” was not listed on the album “London Calling,” which was released in 1979.

According to Kosmo Vinyl, who was part of the punk band’s management team, “Train in Vain” was written and recorded in less than 24 hours after the British weekly New Musical Express agreed to distribute it on a flexi-disc that would be part of the magazine’s cover.

When that plan fell through, the band decided to include the song on “London Calling,” even though the album jackets had already been printed.

“It was not part of any great master plan,” Vinyl says. “In fact, quite the opposite. If the editor at New Musical Express had told me [originally] that he couldn’t do it, the track wouldn’t have been cut.

“At the time, people were saying that the Clash was afraid to have anything commercial on the record, so that’s why they didn’t list it. A lot of people said, ‘They’ve deliberately done this,’ but that’s not true. It happened by chance.”

Twelve years later, however, Nirvana intentionally included a hidden track on its landmark 1991 album “Nevermind,” perhaps starting a trend that has continued into the ‘90s.

Although it never had an official name other than perhaps “The Noise Jam,” the track that begins playing some 10 minutes after the album’s last listed track fades out has come to be known as “Endless, Nameless,” according to author Michael Azzerad’s history of the band, “Come as You Are: The Story of Nirvana.”

“The 10 minutes of silence . . . was the band’s way of playing with the CD format,” Azzerad wrote.

The author added that Nirvana leader Kurt Cobain might have come up with the idea months earlier. While sharing a studio apartment with friend Jesse Reed, Cobain had once wound a tape forward almost to the end and recorded himself saying in a scary voice, “Jesse . . . Jesse. . . . I’m coming to get yooooou. . . .”

As they were getting ready for bed, Cobain popped the tape into the stereo, hit “play” and turned the volume down low. Forty minutes later, a voice said, “Jesse . . ,” and Reed sat up startled. “Hey, did you hear that?”

“Hear what?” asked Cobain, smirking to himself in the dark.