THE MODERN CLARINET
The modern clarinet as we know it - excluding, at this point, other systems - is the boehm or French clarinet as opposed to the Oehler (German system). The usual "black bear" variety, no frills, has seventeen keys and six rings. Variations around this model might include the following:
1. "Full Boehm" clarinet with an added key to the right hand little finger group to play low Eb. This model is called the 20 keys 7 rings. The value of such an instrument enables the player to transpose all 'A' parts on the Bb clarinet, or as in the Third Act of Puccini's La Bohème, the solo at number 28 - on the Bb clarinet - descends to the low Eb.
Many years ago, the 'old Italian' players transposed everything on their choice of either the A or Bb clarinets. They were very skilled in Solfège, were able to read quickly, and not particularly troubled by the transposition up or down a half step. See figure #1.
Figure #1
2. The additional left little finger Eb(G#) key. The duplicates the action of playing Eb fourth space to C third space with either the little finger of the right or left hand. It take a bit of practice, but is well worth it. This key is standard on many models of clarinets produced by Buffet, Selmer, LeBlanc and others.
3. The "Articulated G#" key allows the player to make a very good trill from F# to G# - this trill and its intonation isn't particularly good on the traditional clarinet. When playing the Bizet's opera Carmen, the Second Act, Entr'acte solo on the Bb clarinet has an F#/G# trill; the articulated G# makes it easy. The other solution is to play the solo on the 'A' clarinet and transpose up one half step avoided the F#/G# and playing G/A - much easier.
Summary
The more mechanism added to the clarinet, the more possibility of intonation problems. If it is worth the risk and cost to own a clarinet with "full capability" only to play the few passages of particular concern, then by all means buy it. If transposing everything on the Bb is your challenge, then purchase the "Full Boehm" clarinet, which by the way, comes with - in most cases - the articulated G# and other bells and whistles. Most players prefer the 17/6 and get on with it; the questionable passages taken in stride and are overcome one way or another.