The orchestra

That description fits the BBC National Orchestra of Wales in Cardiff, the City of Birmingham Symphony Orchestra, the Royal Scottish National Orchestra in Glasgow, the London Symphony Orchestra at the Barbican and the Adelaide and Atlanta symphony orchestras equally well. The instrument list barely changes from one country to another. What changes is who pays the players, how many weeks a year they work, and what the hall does to the sound.

The four sections, and how the numbers fall

The four sections are not four equal blocks. Strings dominate by design, because a single violin cannot balance a single trumpet and sixteen of them can. The numbers below describe a full-size orchestra playing Elgar or Rachmaninov. A Haydn or Mozart programme uses perhaps half of them, and a Mahler symphony pushes past the top of every row.

Within the strings, a first-violin section in a full orchestra is normally 14 to 16 players, with the second violins slightly fewer, then violas, cellos and a back row of double basses. The individual instruments, their ranges and what each is normally asked to do are set out separately.

How the orchestra reached its present size

The orchestra grew because composers kept writing for more people, and the people had to be paid. Joseph Haydn wrote 104 symphonies, most of them for the Esterhazy court orchestra of about 25 players, where he spent nearly thirty years; but No. 1 predates that appointment of 1761, and the last twelve, Nos. 93 to 104, were written between 1791 and 1795 for Johann Peter Salomon's much larger London orchestra after Prince Nikolaus died in 1790. Beethoven added trombones, piccolo and contrabassoon to the finale of his Fifth Symphony in 1808 and put a chorus into the Ninth in 1824. Berlioz and Wagner enlarged the brass again. By the time Gustav Mahler finished his Eighth Symphony the forces were large enough for an impresario to nickname it the Symphony of a Thousand, and Mahler left nine numbered symphonies, an unfinished Tenth and Das Lied von der Erde behind him.

Gustav Holst wrote The Planets between 1914 and 1917 for an orchestra of that late-Romantic scale, seven movements running about 50 minutes, ending with a wordless offstage female chorus in Neptune. Nothing much larger has become standard since. The twentieth century mostly wrote back down towards the size of orchestra that halls can afford and boards can employ, and the 80 to 100 figure has held for roughly a century.

Who employs the players, and what self-governing means

British orchestras are owned in three different ways, and the difference decides almost everything about how they work.

• Self-governing orchestras. The players are the members and elect a board from among themselves. The London Symphony Orchestra was founded on exactly this principle in 1904, when players left the Queen's Hall Orchestra in a dispute over sending deputies to concerts. Self-governing players are paid per engagement rather than salaried, which makes the diary dense and the schedule unforgiving.
• Contract orchestras. The players hold salaried contracts with an employing body. The BBC National Orchestra of Wales, founded in 1928 as the BBC Welsh regional orchestra and now based at BBC Hoddinott Hall in Cardiff, is salaried by the BBC and is both a broadcast orchestra and a concert orchestra. The BBC Symphony Orchestra in London, founded in 1930, works the same way.
• Opera and ballet orchestras. These are employed by the company and spend most of the year in a pit. Welsh National Opera, founded in 1943 and resident at the Wales Millennium Centre, carries its own orchestra and its own chorus, and takes both on tour.

Conductors sit outside all three arrangements. They are almost never salaried by the concert-giving body. A guest conductor is engaged for one set of concerts and paid a fee; a music director signs a contract for an agreed number of weeks a year, appoints players, plans seasons and conducts more of them than anybody else.

What a week actually looks like

A professional orchestra prepares a standard programme in about three or four rehearsals of three hours, and considerably less than that on tour, where the same programme is repeated in a new hall each night. That figure is the reason orchestral players are sight-readers of a standard almost no other musicians need. There is no time to learn a piece in the room. The parts arrive marked, the string bowings are already decided by the leader, and rehearsal is spent on balance, ensemble and the conductor's particular reading rather than on notes.

Owain Arwel Hughes, who worked as associate conductor of the Philharmonia Orchestra and as principal associate conductor of the Royal Philharmonic Orchestra, and who was associate conductor of the BBC Welsh Orchestra from 1980 to 1986, built a recording catalogue on exactly this discipline: the complete Vagn Holmboe symphonies with the Aarhus Symphony Orchestra for BIS were taped between June 1992 and January 1996.

Where the rest of this section goes

Five further pages take the machine apart. The sections of an orchestra explains what strings, woodwind, brass and percussion each do musically, and where they sit on the platform and why. Orchestral instruments goes family by family and names every standard instrument, its range in plain terms and the job it is normally given. Symphony orchestra or philharmonic orchestra settles the commonest question of all, which is whether those two words mean different things. The leader describes the principal first violin, the one player whose authority is not the conductor's. What is a symphony? deals with the form the whole apparatus exists to play.

The orchestra is the only large ensemble in Western music that has kept the same basic shape for a hundred years while everything around it changed. It survives because the balance works: sixty strings, a dozen wind, a dozen brass and a handful of percussion turn out to be an instrument in their own right, and orchestration, the craft of writing for it, is still taught as a separate discipline from composition itself.

Also in this programme

• Orchestral instrumentsThe instruments of the orchestra, family by family
• What is a symphony?What a symphony is, and how it is built
• Welsh National OperaA company, an orchestra and a chorus