A few years ago, I was at a pub quiz with a few mates in which one of the rounds involved filling in a sheet with the top 20 best selling albums of all-time.

Now I can’t pretend to be an expect on such matters but about half an hour later, when the scores were totted up, our team had shot up from about tenth of the 18 teams competing to third – and it was all down to my efforts in that particular section.

While virtually everyone competing was aware ¬†Michael Jackson’s Thriller was No. 1, that Queen’s Greatest Hits was right up there ¬†and the vast majority had Pink Floyd’s Dark Side Of The Moon in their selections, the lack of rock music knowledge in the room counted against them.

And so while I knew that Pink Floyd’s The Wall actually outsold DSOTM worldwide, most didn’t.

My list included albums such as AC/DC’s Back In Black, Led Zeppelin IV, Fleetwood Mac’s Rumours, Boston’s debut album, Bat Out Of Hell by Meatloaf, The Eagles Greatest Hits, Def Leppard’s Hysteria and even the more recent Jagged Little Edge by Alanis Morrisette.

I also remember, strangely enough that Shania Twain’s country rock album Come On Over was in the list too.

But this isn’t about what I know, it’s more about what so many people who profess an interest in music don’t know.

Many in the audience that night expressed surprise and dismay in equal measure that Take That weren’t in that top 20 list, nor were Oasis, the Stone Roses or even the Bay City Rollers (!), Duran Duran (!!) or the Osmonds (!!!).

The fact is that to sell millions upon millions of albums, hitting the big-time on these shores isn’t enough.

Nor were regular appearances on Top of the Pops in its heyday (Slade, Sweet) or appearing regularly in gossip columns (Rolling Stones).

One of the easiest bets I ever won was when someone refused to believe that Zeppelin had sold more albums than the Stones. I would imagine millions more share his ignorance.

Rock music, in so many respects, is the world’s best kept secret. Rush are touring the world playing to sell out arenas everywhere but a tiny percentage of people who claim to be music lovers could name more than a couple of songs by the Canadian trio.

The four members of Pink Floyd could probably have walked down Oxford Street when they were at the peak of their powers and be unrecognisable to most passers-by.

And Zeppelin’s refusal to issue singles meant they always were regarded as something of a cult band to the mass media.

Witness for example the coverage of John Bonham’s death to John Lennon’s in 1980. Without for one moment suggesting, the former Zeppelin drummer was as big as the ex-Beatle nor that his death came in such headline grabbing circumstances, from what I recall there was barely a ripple of a reaction in the papers or on TV.

Bizarrely, Gary Moore’s tragic death this week provided the papers with more column inches than that of his former Thin Lizzy colleague Phil Lynott 25 years ago.

I can still recall the headlines when Lynott died in January 1986 with most papers calling him the son-in-law of Leslie Crowther (then one of TV’s most high profile celebrities) rather than the frontman of a famous rock group.

A quarter of a century on and the brand Thin Lizzy is now regarded as sufficiently eyecatching to splash with.

Classic Rock never was, and never will be mainstream and in many ways, that’s the way it should be.

Some artists such as U2, Springsteen ,Queen and even AC/DC managed to appeal to the masses without really compromising their beliefs.

Others – I’m thinking Rush, Bad Company, Scorpions – sell almost as many records but are barely known to a wider audience.

There is no hard and fast rule and the line that distinguishes the mainstream from the cult, remains invisible. Of today’s bands, Kings Of Leon crossed it, The Answer remain very much to the left of it.

Thankfully, genuine rock fans don’t measure bands by popularity or fashion. We judge them purely by the output of their music.

And that’s why the top 20 albums of all-time contain a high proportion of music which can loosely be labelled classic rock.

Ian Murtagh