The first use of the PR rating was in 1996, when AMD used it to assert that their AMD 5x86 processor was as fast as a Pentium running at 75 MHz. The designation "PR75" was added to the chip to denote this.
Later that year, Cyrix also adopted the PR rating system for its 6x86 and 6x86MX line of processors. These processors were capable of handling business applications under Microsoft Windows faster than Pentiums of the same clock speed, so Cyrix PR-rated the chips one or two Pentium speed grades higher than clock speed. AMD did likewise with some versions of their K5 processor, but abandoned the system when it introduced the K6.
The PR rating system drew heavy criticism, some of which seems warranted. The ratings were based on a limited set of benchmark suites which measured only integer performance, which the K5 and the 6x86 in particular excelled at. Both processors had weak floating-point (FPU) performance, far below that of a Pentium. Many experts argued that this made the PR-rated chips poor choices for games, any kind of streaming video, or encoding MP3 music; others took the opposing view that (a) the great majority of users at that time were performing integer-intensive tasks like word-processing, spreadsheeting and web browsing, and (b) the substantially lower cost of the PR-rated processors allowed the user to afford a higher-spec part in any case. The question remains controversial to this day.
With the demise of the Cyrix MII (a renamed 6x86MX) from the market in 1999, the PR rating appeared to be dead, but AMD revived it in 2001 with the introduction of its Athlon XP line of processors. The convention's use with these processors (which are rated against AMD's earlier Athlon Thunderbird cpu core) is less criticized, as the Athlon XP is a capable performer in both integer and FPU operations, and manages to out-perform an Intel Pentium 4 at a PR rating equalling the P4's mhz.
In any case, both raw MHz ratings and the PR scheme are essentially marketing tactics aimed at the naive consumer. Most professionals or even interested amateurs now consult extensive benchmark tests to determine system performance on various application if performance is actually a consideration in system purchase.