QUOTE(sarah-flute @ Sep 24 2005, 09:07 PM)
Amanda, the two links Storini posted above were really interesting/useful/informative. I am guessing also that half double would be another name for compensating, as from what I've read for the compensating ones, they have more tubing than a single and can do more notes etc like a double, but instead of basically one whole horn in F and one whole horn in Bb, you have a horn in... Bb I think... and then it has extra tubing for F so that when you play in F you're using the Bb tubing plus some. Whereas for a double horn you use an entirely different set of tubing.
Did that make ANY sense??!

Sounds good to me.
Horns come in a large number of different designs, and nearly all of them cover all the range that you would need up to about Grade 6 or 7. Some milestones in their history:
16th to early 18th C - one-piece, therefore only capable of the harmonic series in one key (often D or Eb)
mid-18th C - crooks between mouthpiece and body, to allow variation of length and therefore basic key. Horns of this type become regular members of the Classical orchestra, usually two, occasionally four.
18th C (probably, exact date not known) - players use right hand in bell to obtain extra pitches and tune the notes of the harmonic series that don't occur in the conventional scale. This technique is used widely by soloists, sparingly by orchestral players.
late 18th C - cor solo invented, without end crooks, but with tuning crooks of different lengths inserted into the body tubing.
Early 19th C - right hand adjustment becomes more widespread.
1820 to 1835 - valves invented, both rotary and piston variety, allowing rapid insertion and removal of extra tubing. Three of these give full chromatic range. Orchestral horns still have end crooks, and composers still expect players to use them in addition to the valves.
1898 Kruspe markets the first double horn. A more satisfactory design followed in 1900. The double horn is based on the cor solo, with mouthpipe fixed to the body.
During the 20th C, horn designs have multiplied prolifically. The standard full double has a length with all valves open (i.e. not depressed) that gives the F harmonic series. Depressing the thumb lever diverts the air flow to give the Bb above. Whichever path the airflow takes, it goes through the finger valves once, and dperessing a finger valve inserts extra tubing of a length appropriate to the thumb valve position. The effect is to flatten the note by a tone (1st valve) a semitone (2nd valve) or a minor third (3rd valve). A compensating horn also has a thumb valve that changes the basic length from F to Bb, but now while the airflow on the Bb side is routed through the finger valves once, as above, on the F side it goes through twice. The two added portions add up to the same length as the F side of the corresponding valve on a full double.
Other designs that have become available during the last 50 years:
Bb and F alto - all valves open gives a Bb horn; thumb valve down gives the F above.
Bb single - these are available with three, four and five valves. The first extra thumb valve usually comes with two lengths of slide, one used for hand stopping, because none of the finger valves on a Bb horn is the right length to adjust for the pitch change, the other giving the length of the F horn. The five-valve single has both these lengths.
Beginners are often advised to start on a compensating horn in F and Bb. The danger with a single Bb is that it does not encourage the player to develop a good sound. Compensators are lighter than full doubles, and can be very good instruments.
Horn students need horn specialist teachers. The horn is unlike the other brass instruments, partly in its in-built character, partly in its very different history, to the extent that few generalists can give the whole picture, and it is easy for a beginner to get into bad habits, especially with his/her embouchure.
If you want to practise the horn quietly, you can use a mute with an electronic system by Yamaha, called Silent Brass, that damps the external sound to almost zero, but allows you to hear what you are doing through earphones. The price is somewhere over £100, I believe.