Metallica had been on the brink of breaking through to the mainstream for a few years. They even scored a minor hit in the form of "One" from 1988's �And Justice For All. But it wasn't until the epic opening section of "Enter Sandman" crept onto radio in July 1991 that the band really exploded into the mainstream. Likewise, Nirvana had been around since 1987 and had sold 40,000 copies of their debut album Bleach in 1989, but Nevermind really busted things wide open for the band.

Like a huge rock dumped in the path of a small stream, the release of the Black Album on August 13 and Nevermind on September 24 seemed to split the direction of guitar playing - and therefore rock music in general � into two distinct paths. One side - the Nirvana-influenced side � favored rougher technique, grittier tones and relatively minimal production. The other side, championed by Metallica acolytes, preferred a heavier, more technically precise sound. The rise of grunge is well documented, and while Nirvana's success helped open the door for their contemporaries, it also opened the floodgates to a surge of imitators. Similarly, the bottom-heavy sound of the Black Album can be heard as an influence on many later recordings, from the immediate aftermath to now, twenty years later.

Black Album producer Bob Rock seemed to take some of the Metallica guitar tone with him when he produced Motley Crue's self-titled album in 1994: the single "Hooligan's Holiday" prominently displays a very Metallica-like rhythm guitar tone in its chorus. Anthrax's The Sound Of White Noise features similar high production values and song structures; and Testament's The Ritual exhibits similar structural streamlining and an increased focus on melody, as did Megadeth's Countdown To Extinction. Did these bands change their approach because that was the way metal was evolving, or were they encouraged by their labels to veer more towards the formula that worked so well for Metallica? It's impossible to say, but intriguing to ponder. It's undeniable that after Metallica led the way, metal became much more mainstream. Pantera's Far Beyond Driven debuted at #1 in 1994, while a year earlier Morbid Angel became the first death metal band to sign with a major label (Giant, a joint venture of Warner Bros. Records and Irving Azoff).

As for Nirvana, Kurt Cobain's distinctive vocal delivery and Pixies-inspired quiet verse/distorted chorus songwriting style were echoed a couple of years later by bands such as Bush and Silverchair, then on through Lifehouse in the early noughties and to modern bands like 30 Seconds To Mars. In fact 30STM vocalist Jared Leto is a declared Nirvana fanatic � he performed Nirvana's "Pennyroyal Tea" dressed as Cobain on the 17th anniversary of Cobain's death.