Is it Mick Jagger's band? Is it Keith Richards'? Is it a creature of the two of them, the feuding but inseparable Glimmer Twins with a legacy of songs second only to Lennon and McCartney's? Fifty years after the birth of the Rolling Stones, the question remains unanswered. The band that began playing as The Rollin' Stones fifty years ago in London on July 12, 1962, is still the global icon of what marketers and journalists who can't think of a better phrase call "the rock and roll lifestyle." And it's still fronted by the two former schoolmates who started talking about blues records on a train in England in 1960, and found out they had a common interest in American music that's lasted to this day.

That interest in the blues, and more generally in the music of black Americans that was only then becoming acceptable for white audiences, is what gave the Stones their unmistakable sound. Listening with the devotion of fanatical teenagers  to everything from Jimmy Reed to Chuck Berry, Jagger and Richards absorbed the guitar-driven sound of electric blues and reproduced it faithfully. The sharp twang of Richards' guitar recalled closely Berry and Hubert Sumlin, who had epitomized the Chicago blues, and rapidly became the Stones' trademark.

Richards built on the legacy of his idols to perfect a guitar sound that spawned countless imitators, and he continues to do so today. "He invented a style of playing that was unprecedented, much like Ray Charles did on piano," said blues guitarist Tinsley Ellis. "He is one of the most imitated guitarists in rock and roll."

As much as the music, it was Richards' life -- the drugs, the legendary tours, the swirl of beautiful women and untold amounts of money that characterized the excesses of rock and roll's 1970s heyday -- that captivated audiences. But to critics, he has always been a guitar legend rather than a world-class partier. That's what propelled him to number 4 in Rolling Stone's 2011 list of the best rock guitarists of all time.

Aside from his unique sense of timing, and his interaction with drummer Charlie Watts, who according to Richards himself is the true soul of the band, there's another trick behind the Keith Richards sound: his guitar is tuned differently from most.

On many of the Stones' hits, Richards used an open-G tuning, meaning that his guitar's strings were tuned to G-D-G-B-D from lowest to highest, with the top one removed, instead of the usual E-A-D-G-B-E. He had learned it from American guitarist Ry Cooder, who in turn had taken it from old blues masters: an approach to the guitar that  Richards himself famously described as "five strings, three notes, two fingers and one a---hole to play it." (The open-G tuning can be heard on songs including Brown Sugar, Honky Tonk Women and Start Me Up, among many others. "It's actually a five-string banjo tuning that dates back to when the guitar began to replace the banjo in popularity after the First World War," Richards told one interviewer, according to Keith Richards: The Biography, by Peter Bockris. "It's called open tuning, or the Sears Roebuck tuning sometimes, because they started selling guitars then." )    

But the Stones were always a two-guitar band, and sometimes three, with Jagger joining in on songs such as the disco hit Miss You and the raunchy When The Whip Comes Down. So who was the best guitar partner for Richards?

Original member Brian Jones was a talented multi-instrumentalist whose facility with non-traditional instruments, such as the sitar, marimba, or mellotron, gave the band new horizons to explore in the 1960s. But he was a self-destructive, abusive character with six children from six different women, and he ended up being fired from the band a month before his death in a pool accident in 1969.

Hardcore Stones fans swear by Mick Taylor, who joined the band after Jones and stayed until 1975, when the toll of a fast, drug-fueled lifestyle proved to be too much for the young British blues guitar phenomenon. His fluid, dazzling soloing, reminiscent of Eric Clapton, had relegated Richards largely to rhythm guitar but contributed to a sound that identified the most creative and musically substantial period of the band's history. Witness the 1973 double album Exile On Main Street, regarded as the best rock and roll record by many of their fellow musicians, or Can't You Hear Me Knockin' on Sticky Fingers.

In his place, the Stones recruited Ron Wood from The Faces, Rod Stewart's old group -- and what they lost in musicianship they gained in flamboyance, and in Wood's ability to match Richards in debauchery. Those were the worst years of Richards' heroin addiction, which almost landed him in prison for decades after he was caught in 1977 in Toronto with vast amounts of the drug. But they were also the beginning of a new musical phase for the band, with Richards and Wood trading (miraculously, it seemed, while often barely conscious onstage) guitar licks in an unusual back-and-forth form that Richards calls "the ancient art of weaving." He had done something of the sort with Jones as well, but it was with Wood that the Stones' two-guitar style truly came into its own.

That's the sound exemplified by ballads such as the 1978 hit Beast Of Burden, with neither guitarist playing a clearly defined rhythm or lead, just the two of them riffing off of each other to the steady beat of Watts and the pulse of bassist Bill Wyman. To many, that is the classic Stones lineup. (It's not, in fact, the one that has survived the longest: that distinction belongs to the current one, featuring Darryl Jones on bass since 1993.)  

But it's Richards who has given the Stones the distinctive, guitar-heavy sound that has been their trademark for fifty years, along with Jagger's vocals. "The others are all excellent. But only Keith is irreplaceable", said Ellis.