The sections of an orchestra

The four sections are unequal on purpose. Strings are about two thirds of the players, because a violin is a quiet instrument and a trumpet is not, and balance has to be built into the numbers before a conductor can do anything about it.

What each section is for

Strings: the continuous line

The strings carry the melody more often than everyone else combined, and they also carry the harmony underneath it. A violin, viola, cello or double bass can hold a note for as long as the bow lasts and then change bow without a seam, so the string section is the only part of the orchestra that can sustain sound indefinitely. That is why composers give the strings the long paragraphs. Ralph Vaughan Williams wrote the Fantasia on a Theme by Thomas Tallis in 1910 for double string orchestra and a solo quartet, with no wind at all, and the piece works because sustained string sound alone can fill a cathedral.

Woodwind: colour and character

The woodwind section supplies solo voices and the inner colour of chords. Each woodwind instrument sounds unmistakably like itself, which is exactly what the strings do not do, so a composer who wants one line to step out of the texture gives it to an oboe or a clarinet. Woodwind players almost always play one to a part, which means each of them is a soloist by default and is exposed in a way no rank-and-file violinist ever is.

Brass: weight and structure

The brass mark the structure. A horn call or a trombone entry tells a listener, without any analysis, that something in the argument has changed. Brass can also simply be loud, and the loudest passages in the repertoire are brass-led: William Walton scored Belshazzar's Feast in 1931 for baritone, large chorus, orchestra and two extra brass bands, and the effect at the fall of Babylon depends entirely on that reserve of unused brass.

Percussion: time and punctuation

The percussion keep and break the time. Timpani are the oldest members and the only tuned drums in standard use, and a timpanist is treated as a principal in their own right. The rest of the section, side drum, bass drum, cymbals, triangle, tam-tam and the tuned metal and wood, is used sparingly and therefore lands hard when it is used at all.

The four sections in numbers

The four sections divide as follows in a full orchestra of about 80 to 100 players, the size needed for Elgar, Rachmaninov or Holst. Smaller repertoire uses a fraction of it.

Inside the strings the numbers descend from the front: 14 to 16 first violins, slightly fewer seconds, then violas, cellos and a back row of six to eight double basses. Every one of those players reads from a part shared with a desk partner, and only the two of them turn the page, one playing while the other reaches.

Where each section sits, and why

The seating plan is an acoustic decision, not a hierarchy. The layout used by most orchestras today puts the strings in a fan across the front, first violins on the conductor's left, then second violins, violas and cellos moving clockwise, with the double basses ranked behind the cellos on the right. Quiet instruments are at the front because they need to be; loud ones are behind because distance and height do the balancing.

Behind the strings, the woodwind sit in two rows in the centre, flutes and oboes in front, clarinets and bassoons behind, close enough to hear one another tune a chord. The brass go behind the woodwind and slightly to the side, horns usually to the left, trumpets and trombones to the right, so that the horns can face across the platform rather than blow straight into the audience. Percussion and timpani occupy the back wall, where their sound has furthest to travel and therefore reaches the audience fractionally later than the front desks do, which is why back-row players learn to place a note a shade ahead of the beat. The harp, when there is one, sits at the side, usually on the left, because the instrument is played facing inwards.

Some conductors use the older antiphonal layout instead, with the second violins on the right facing the firsts. Music written before about 1900 often throws a phrase from one violin section to the other, and that effect disappears entirely when both sections sit together. The choice is audible, and it is one of the things a conductor settles at the first rehearsal.

How a section is organised

Every section has a chain of responsibility that operates whether or not a conductor is present.

• The leader is the principal first violin and the senior player of the orchestra: tunes the ensemble, decides the bowings for all the strings and plays the violin solos.
• Section principals sit at the front of each string section and at the first desk of each wind and brass group, play the exposed solos, and set the manner of playing behind them.
• Sub-principals sit next to the principal, cover the part when needed and take the divided upper line when a passage splits.
• Rank and file players fill the remaining desks, and in the strings the requirement is uniformity: same bow, same length of note, same vibrato.

The additions, and what they signal

Harp, piano, celesta and organ sit outside the four sections and their presence tells a listener something before a note is played. A harp on the platform means late Romantic or twentieth-century repertoire. A celesta usually means a specific effect somebody wanted. An organ means either a very large work or a hall built with one, and St David's Hall in Cardiff, opened in 1982 with 2,000 seats, has an instrument prominent enough that the Welsh Proms built a whole Organ Prom around it.

The four-section division is old enough to be invisible, and it survives because it is not arbitrary. Strings sustain, woodwind characterise, brass weigh and percussion punctuate, and a composer who understands those four verbs can write for an orchestra without ever seeing one.

Also in this programme

• The orchestraHow a symphony orchestra is put together
• Orchestral instrumentsThe instruments of the orchestra, family by family
• What is a symphony?What a symphony is, and how it is built
• Two ensembles, one founded and one inheritedCamerata Wales and the National Youth Orchestra of Wales