As with the woodwinds, the brass section has several parts per type of instrument and they are grouped usually in pairs in the same stave. To make clear what line is played by which instrument, they are marked with the following indications:
1., 2., 3., etc.
A number state which player plays which line when either there is only one line at a time, two that cross with each other, or any other reason that makes the music not clear to the conductor. Since parts are usually separated by player such a confusion doesn’t exist. Numbers are placed above the stave, unless it needs to be clearer and one of the numbers goes below and besides the line it refers to.
When a single line is only played by one instrument, it has more individuality, like a story or a song that is sung by one person, or the dialogue between two or three individuals as opposed to a group or choir singing altogether. The dramatic effect is totally different. It is also a way to give more subtlety and delicacy to a melody or contrapunctual line.
A2, A3, A4, etc.
Used even more than in the woodwind family, the terms “a” immediately followed by a number states the number of players that play that line. Unlike the strings and woodwinds, a pair of French horns sound very good in unison. This also applies when a bigger number of horns play a chord together. For example, in an instrumentation of 6 horns, these will usually play three notes in the chords, 2 per note.
Trumpets and trombones are also used in pairs, though not nearly as much as the horns. Unisons in a3, a4, a6, a8, or as many as we can afford, are used extensively through the big symphonic repertoire of the 20th century.
When using a number of instruments in unison, the reason for using French horns in pairs (a2) is to give a little more body and dynamic to each line so they can compete fairly with the rest of the brass instruments. While the purpose, though, of using a3 or a4 in brass instruments is to change the actual sound quality of the line, as well as to significantly increase the dynamic power. Just as when two different instruments of the same or different family blend in unison into a hybrid of the two, when some of the same brass instruments play in unison they blend into a new brassier, heavier and more epic version of the instrument.
Examples of horns a2, a3, and a4 passages in G. Holst's The Planets: Jupiter
Other languages: FR
Examples of trumpets 1., 2., a2, a3, and a4 passages in G. Holst's The Planets: Jupiter
Other languages: FR