The Orchestral or French Horn (really a misnomer) is normally just referred to as the Horn. Most modern horns are double instruments pitched in F/Bb. See below. They are actually much longer in their basic tubing than similarly pitched brass instruments, some12 ft, as they are played in higher harmonics than most brass instruments, and are harder to play. They utilise a funnel-shaped mouthpiece and have 4 rotary valves operated by the left hand. The right hand is placed inside the bell. Single horns with 3 valves, and pitched in F or Bb are commonly used by younger players. The horn plays in the orchestra and the military or concert band, but not in the brass band.

Wagner Tubas in F or Bb are essentially relatives of the orchestral horn and played by horn players. Built in upright oval form, they have a much narrower bore than real tubas, and are played with funnel, not cup, type mouthpieces. They use rotary valves operated by the left hand (unlike superficially similar German style oval tenorhorns and baritons, which are right handed). Only used orchestrally. Not likely to appear very often on eBay!

Orchestral Double Horn (left) and Wagner Tuba (right)

Eb/F Tenor Cor or Mellophone - appears in a variety of shapes, often like a large flugel horn or coiled like the orchestral horn. Unlike the horn, the tenor cor has a much shorter length of tubing, the same as the tenor horn, and is played with the right hand, not the left. The other hand is not placed in the bell during playing. The tenor cor is usually equipped with Perinet (piston) valves rather than rotary valves. This horn is sometimes used in the concert band instead of the French horn, or in marching bands, but it is not commonly used in the U.K. 3 valves. Approximately 6-7ft of tubing (excluding valve tubing). See below.

  Eb/F Tenor Cor

Eb Tenor Horn or Tenor Saxhorn increasingly known as Eb Alto Horn to match up with American and European naming. The standard brass band tenor horn. 3 valves. Approximately 7ft of tubing (excluding valve tubing). Style varies considerably - see photos below. There are also some bell-front marching versions resembling large flugel horns. 

Eb Tenor Horn (traditional saxhorn form)

Left - Eb Tenor Horn  and Right- Bb Baritone in shorter open wrap 'tuba form', with high upper bow

Bb Baritone (Horn) or Baritone Saxhorn, the standard brass band baritone; more or less equivalent to the German Bb Tenorhorn (the latter is normally constructed in oval form). Usually 3, but sometimes 4 valves. Approximately 9ft of tubing (excluding valve tubing). Style varies in the same way as with tenor horns. Above right and below left. Just to complicate matters, Americans refer to basic 3-valve euphoniums as 'baritones' and refer to the baritone saxhorn as the 'British baritone'! The baritone has the same pitch as the two instruments below, but it has a lighter tone than the euphonium, closer to the trombone in sound.

Bb Bass Saxhorn - largely obsolete, but still used in French military bands alongside the euphonium. Courtois still manufacture them with four or five valves. The now obsolete orchestral French Tuba in C is a six-valved version. The bass saxhorn resembles a compact euphonium, and has a low upper bow and a tuning slide between the leadpipe and the valve block. It is wider bored than the baritone. In sound, it lies between the baritone and the euphonium, and in a band tends to blend more easily with the woodwind than the euphonium. 3, 4, 5 or even 6 valves! Approximately 9ft of tubing (excluding valve tubing). Below centre.

Euphonium or Tenor Tuba, normally in Bb or Bb/F. .The smallest of the tubas, and wider bored than the saxhorns. Usually nowadays has a tuning slide beyond the valve block, creating better intonation. Produces a broader and mellower sound making it ideal as a solo instrument in the tenor/bass range. Standard in brass, military and concert bands; occasionally found in the orchestra. 3 or 4 valves. Some older models have 5 valves. Approximately 9ft of tubing (excluding valve tubing). Below right.     

                                   Baritone / Bass saxhorn / Euphonium compared                                          

A German-style Kaiserbariton (a euphonium built in oval form, and with rotary valves)

Tubas (Simply ‘Basses’ in the brass band). Bass tubas are built in F or Eb and Contrabass tubas in C or Bb. Double letters, e.g. EEb, tend to indicate large-bored instruments with extension down to the fundamental note. 3, 4 or 5 valves. Standard in bands and orchestras. 12 to 18ft of tubing (excluding valve tubing). Tubas vary greatly in form, some point the bell to the right and some to the left. Some have piston valves, others have rotary valves.

Piston and rotary valved tubas

Helicons and Sousaphones are coiled tubas which are carried over the shoulder and around the body. Really only marching band instruments. Sousaphones differ from helicons in having a 'gooseneck' angle near the top of the bell, and usually a very much larger bell.

Illustration of Sousaphone (left) and Helicon (right)

Trombones need little introduction except to say that they are mainly tenors in Bb or Bb/F, or wider bored basses in Bb/F. Essentially a tenor trombone is a bass slide trumpet! The dual pitch trombones have additional rotary valve(s) operated by the left hand as well as the slide operated by the right hand. Old brass band bass trombones were pitched in G, and the slide was fitted with a handle. There are smaller and rarer Eb alto and even Bb soprano trombones. Occasionally trombones are fitted with valves instead of the slide.

Bb/F Bass trombone

The Cimbasso (below) is a contrabass valve trombone, usually in F or Bb, and originating in Italy. Its pitch and range is similar to that of the tuba, but its tone is more strident. Usually 4 or 5 rotary valves. Not commonly used and infrequent on eBay.