What began as fondly jamming to the post-Ozzy Osbourne Black Sabbath tracks "Heaven and Hell" and "The Mob Rules" during Sabbath's Ozzfest 2005 soundchecks eventually led to a reunion with former sing

What began as fondly jamming to the post-Ozzy Osbourne Black Sabbath tracks "Heaven and Hell" and "The Mob Rules" during Sabbath's Ozzfest 2005 soundchecks eventually led to a reunion with former singer Ronnie James Dio, according to bassist Geezer Butler.

"We just thought it was a bit of a shame that we can't play those songs again," he tells Billboard.com. "And then earlier this year [Sabbath guitarist] Tony [Iommi] went to see Dio. They wrote a couple of songs together and it just went from there."

Not only will an upcoming Dio-era Sabbath anthology feature three new songs ("Shadow of the Wind," "Devil Cries" and "Ear in the Wall"), but, as previously reported, a world tour is planned for 2007 as Heaven And Hell, the title of Sabbath's first post-Ozzy album from 1979.

"The [Dio-era had] really good songs [that were] a lot different to the old original Sabbath stuff," Butler says. "They really are fun to play because a lot of them have a lot of space, where you can improvise a lot instead of sticking to the rigid formula."

For Dio, who hasn't performed with his old bandmates since 1992 and never stopped playing his era of Sabbath tunes, the reunion wasn't solely about walking down memory lane.

"The writing process was so much fun and harked back to how productive Tony and I were as writers before, so I think that's what kind of kick-started everything," Dio tells Billboard.com. "I think for the other guys it's more of a revisiting that pleasure perhaps, but it wasn't a big thing for me. But it will be nice to play them with the original guys again. That will be special."

Current touring plans call for a 10-date Canadian run with drummer Vinnie Appice (who appeared on the band's 1992 album, "Dehumanizer") this spring followed by a few New York shows. From there, Heaven And Hell will head around the globe, including European summer festival dates. The outfit hopes to return Stateside in the fall for a full tour. For now, there is no talk of a full-length album, with Dio saying he hopes to release another solo set in 2008.

The media has portrayed the Heaven And Hell announcement as a bargaining tool to get Osbourne into the studio for a long-awaited new Black Sabbath album. However, Butler discounts this notion.

"We did try it in 2001 and I think we tried to do too much," Butler said. "We went for quantity rather than quality. I think [Ozzy] and Tony need to sit down together more, like the way Ronnie did with Tony this time. You don't need a whole studio or anything."

He adds, "I think we are talking about one final Sabbath album and a final Sabbath tour, which will obviously be after next year when we finish the Heaven aAd Hell thing. We'll have to see how things go."