A+

For a live recording, this is the pinnacle of studio perfection.  The benchmark, after more than 10 years, is still Pink Floyd's commercial releases of P.U.L.S.E on CD, cassette, and VHS format (and soon on DVD!)  I won't trade these, but if you want a brand new copy of your own, you can get one commercially.

A

These are the very best recordings of illegitimate origin.  All instruments can be clearly heard, there is an absolute minimum of distortion, and the recording is free of major problems.  These are most frequently soundboard tapes, FM broadcasts, etc.

A/A-

This is a thoroughly enjoyable source, but will have a few minor problems, such as slightly intrusive audience noise.  This is still a recording you could play for someone inexperienced in listening to recordings of illegitimate origin (ROIOs).  Not perfect, but very close.  These are often audience DAT clones.

A-

A very good source, but with a few more problems.  These may include a narrower dynamic range, more intrusive audience noise, minor tape faults and microphone movements, etc.  This is clearly not commercial material, but also clearly a worthwhile recording.

A-/B+

These sources could possibly have some major faults.  Problems may include cut tracks, more severe tape distortion, excessive LP surface or audience noise, and other issues.  The typical vintage audience ROIO sound.  Still listenable, but will take a bit of imagination.

B+

Major problems, but still a listenable recording for the properly motivated.  Probably to be avoided if it would be one of your first acquisitions representing the group.  Those inexperienced listening to ROIOs might not be able to tolerate one of these for very long.

B

Fairly dire.  Blatant occurrences of many of the problems detailed above.  Even patient listeners may soon protest.  Really only for the more hardcore collector, or for those with a personal interest in the date and place.  Anything below this rating fits that description also.

B-

For the not only hardcore, but also masochistic collector.  Sounds like a tape made wrapped up in a winter coat under the seats in the back row.  The protests of less patient listeners could become violent.  Some sensitive individuals could consider these recordings to be an instrument of torture.

C+

If played too loudly, the terribly bad quality of these sources could provoke a neighbor to call in a complaint to the local authorities.  We now begin wondering why this recording even survived.  Bordering on (or in fact) unlistenable.  For the most severely crazed completist.