It is often difficult to tell, in an on-line auction format, just what one is viewing. We all know the basics, and most of us have attuned our eye to differentiate between subtle key differences among clarinets, but there are times when, despite our knowledge, we remain uncertain as to precisely what we've just purchased until it arrives. Ofttimes, sellers know very little about the instruments they possess, and rarely provide high quality photos in abundance. Using the total length of a clarinet is generally a good baseline, but is in no way entirely definitive--especially for clarinets from the early 20th century or before, where bell length and the length of the bottom joint past that last tone hole could vary significantly--both non-critical dimensions.
I have found the best measurement to ask for from a seller is NOT the overall length from barrel to bell, but, in fact, the measurement from the top of the barrel to the DEAD CENTRE of the first left hand index finger tone hole (F#). Because key and pitch are scientific, measurements will nearly always be the same within no more than a 1/8" discrepancy for a specified pitch, (regardless of bore diameter and tone hole size, within "reason", I believe). One can then take the reported measurement, and compare it with clarinets of known key and pitch, and know what is being viewed before making a costly purchase. The above recommended measurement is an arbitrary measurement, of course, but it's extremely consistent. Using this as a guidline, it should then become quite clear if one is dealing with something outside the spectrum like 405-435 hertz (unusual, but extant--especially 432-435hz, prior to WWII) or super high pitch like 460+.
I'll attempt to cover the key of C, as I have several of them to compare, whereas I own no key of A clarinets, and only LP (440-442hz) Bb clarinets. A mathematician with an interest in woodwinds would be the best person for this job, as he or she could develop a scientific graph, based upon sonic standards without needing the actual instruments to do so. Alas, I am not that fellow, but I can provide the following (AMMENDED):
430-432hz = 6 1/8", approx*
440-442hz = 5 15/16", approx*
450-454hz = 5 3/8"-5 3/4", approx*
Notable discrepancies are often found among antique manufacturers' non-standardized products (see 450-454hz, as I have personally recorded that rather significant 3/8" discrepancy over only 4hz), as pitch and consistency is also affected by factors other than just length. Variances in placement of some of the tone holes, tone hole diameter and mouthpiece table length are just a few of the primary factors that can skew the accuracy of the above reference regarding pitch.
*(Approximations are typically, but not always, within 1/8" [3mm] of the median. This covers slight variations or inaccuracies in the manufacturing process or a builder's preference, albeit perhaps erroneous.)
If others would be interested in filling in HP and LP key-of-A and Bb soprano clarinets (and perhaps Eb, too), we might be able to have a sticky thread we could use as a long standing resource.
