The members of Tool had a credo when they formed the band 16 years ago: "substance over style, art over image."

The four men of Tool -- anonymous band members behind vaguely androgynous frontman Maynard James Keenan -- had a credo when they formed the band 16 years ago: "substance over style, art over image."

"We wanted people to get into the music, instead of going, 'Well, how long is their hair?' and 'Are they cute?'" guitarist Adam Jones says. "We just stood in the shadows and worked really hard."

Without ever really leaving those shadows, Tool has quietly become one of the world's most commercially and uniquely successful bands. And it has done so while repeatedly bucking industry convention. Tool often waits up to five years between albums. Its last three singles -— including current hit "Vicarious" -— have averaged more than seven minutes, forcing some radio programmers to create their own edits.

And while the music business clamors to embrace digital formats, Tool has yet to reach an agreement with its label, Volcano/Zomba, for such distribution.

Yet Tool is more popular than ever. The band's new album, "10,000 Days," marked Tool's second appearance at the apex of The Billboard 200, with 564,000 albums sold its first week, according to Nielsen SoundScan. This follows the sale of more than 8 million copies of its first three studio albums in the United States. In the United Kingdom, "10,000 Days" scored the group its highest bow with a No. 4 debut on the albums chart. The title debuted in the top 10 throughout Europe.

Tool fans span a cross-section from metalheads to emo kids, punks to goths, with its arty, prog-drenched heavy rock. And it seems the more the band follows its own vision, the more it underscores its cult-band cool -- it's just a cult of hundreds of thousands at this point.

Zomba Label Group president/CEO Barry Weiss calls the band a "throwback to the old days when Led Zeppelin came out with an album -- everybody bought the album. It's that simple." Tool's success, he says, is fueled by a double effect of true artist mystique and a less-is-more mentality.

"They don't oversaturate their audience," says Mike Stern, VP of programming for Emmis/Chicago. "There's not a record every 14 months and a tour every summer."

The band has long tended to its mystique. Ballooning from the popularity of its early, pioneering videos -- especially the stop-motion animation of "Prison Sex," rising eerily above the masses of grunge and urban pop on MTV in 1993 -- Tool has carefully cultivated a dark image, through album and T-shirt artwork and onstage visuals.

"We've basically used art as a very strong propaganda tool to coincide with the music," Jones says. In general, he says, the band "is just a really cool experimental project that we're all in."

The experimental approach certainly included the packaging for "10,000 Days." The album is configured like a folding book, with one flap carrying stereoscopic lenses, and the other a booklet containing sets of paintings and photographs on each page. When spied through the lenses, each set emerges as one 3-D image.

Retailers often frown on unusual packaging because of increased concerns regarding shipping and display. But call it one more example of Tool flying—high—in the face of industry convention.

Fans love it, says Bryan Everitt, director of music operations for the 153-store Hastings Entertainment chain. "It's great to see music lovers reading the liner notes and really enjoying holding the product in their hands again," he says, noting that the album set the Amarillo, Texas-based chain's record for midnight sales with 5,000 copies sold on the album's release date, May 2.

A proud Jones, who came up with the concept, says, "[Avant guitarist] Robert Fripp was at our show the other day, and he said, 'This is the best album art since the '70s' ... We're always trying to think of something to do that's never been done before. We want people to get more than their money's worth."