Another March, another Rock and Roll Hall of Fame induction ceremony. The inductions, televised yesterday, inevitably inspire disgruntled e-mails and confused calls from music fans, wondering how the Rock Hall defines "rock" and who is rocking enough to belong there.

In 2007, R&B/disco diva Donna Summer and disco kings Chic were nominees, and pioneering Grandmaster Flash and the Furious Five were the controversial first hip-hop group inducted.

The class of 2008 has two controversial choices, beginning with Madonna, a pop-culture icon whose music seldom has much to do with traditional rock.

Leonard Cohen, the 73-year-old erudite poet/songwriter/author/Canadian, also doesn't do much traditional rocking.

The other inductees, all rockers: The Dave Clark Five, John Mellencamp and The Ventures.

Founded in 1958 by Don Wilson and Bob Bogle, The Ventures had such hits as "Walk, Don't Run" and "Hawaii Five-O." The band out of Tacoma, Wash. sold an estimated 100 million records.

Artists become eligible 25 years after their first release, and since 1986 — when the initial class included Chuck Berry, James Brown, Sam Cooke, Elvis Presley, Fats Domino, the Everly Brothers and Ray Charles — the rock hall had a solid decade's run of obvious picks, classic rock legends and influential R&B and country artists that lasted well into the '90s.

But once more contemporary artists such as 1999 inductee Billy Joel began to enter the fray, fans began crying foul and questioning the Rock Hall's dedication to honoring "rock" and the artists who make the music.

Obviously, the traditional three power chords, a steady backbeat and a good hook are not enough to define rock. And, across its 50-plus years, rock music has influenced and absorbed bits and pieces of other styles (hence the many subgenres with "rock" in their name). It has been such a dominant musical form for so long that in most music stores, "rock/pop" is still a common category.

So, how does the Rock Hall Foundation define "rock?"

"We define it fairly broadly," said Jim Henke, the hall's chief curator. "Rock 'n' roll has become the umbrella for popular music. ... When you go back to its roots of country and blues and R&B and gospel and start there, it basically developed out of all these things, and it's evolved so much."

Henke said music is so subjective that everyone has a different definition of what rock is, often directly influenced by what he or she likes, and thus visitors often leave comments questioning the inclusion of musicians who don't easily fall into what could be called "traditional rock" forms. Perhaps more importantly, the artists simply don't resonate with these visitors.

"A few years back, when we had our U2 exhibit, we had people come in and say 'Hey, that's not rock 'n' roll.' Some people think it ended with the Beatles or shortly after the Beatles, some people object to hip-hop and some people object to punk rock, so everyone has their own definition," Henke said.

Moving forward, he said the umbrella will only get bigger: "It's always interesting to hear the debates about these people, like Donna Summer."

Henke added that while record sales are not a direct factor, longevity and musical/pop- culture influence also play a big part in the voting body's choices, and that the music and/or artists should have "some kind of edge to them."

As with many concepts in music and pop culture, "edge" in relation to an artist's image or their work is also debatable.

While many of the early rock 'n' roll, tuxedo-wearing doo-wop and evening-gown-donning girl groups that have been inducted can be given a pass on the edge factor, there are plenty of contemporary artists — like Bonnie Raitt (2000), James Taylor (2000) and Jackson Browne (2004) — who don't immediately conjure up "edgy" when compiling a list of adjectives to describe their music or sound. Nor do the names and works of The Bee Gees (1997) or Earth Wind & Fire (2000).

It wouldn't even be too difficult to find music lovers (including this writer) who find Billy Joel talented and sometimes interesting, but lacking any tangible musical or image-related "edge."

For music fans who still care about the Rock Hall, who is welcomed and when, each year's list of nominees is likely to confuse and/or infuriate them further.

The debate will continue, which is a good thing, because it keeps folks talking about artists and music with passion, instead of just wondering which one of their favorite songs will turn up next in a television commercial.