Instruments of the Orchestra
The backbone of the orchestra has always been a string section. Reading various reports and books always subject, many people seem to be puzzled by this, but as a string player I think it's quite obvious why. String instruments can play with the greatest variety of tonality, expressivity and dynamic range. They are the instruments that can play the most melodious tunes, the most vicious and angry passages, and play harmonies with a tremendous richness. Compare this with the oboe for example, which has a wonderful piercing tone, so piercing that it can stand out in a solo against the rest of the orchestra. A truly wonderful instrument, but who, other than oboe players, would want to listen to an oboe sound for the 2 hours of the concept. Instead, a composer uses an oboe to achieve tremendous effect in short bursts, so even when put together with its deeper relation, the Cor Anglais it wouldn't act as the backbone of a Symphony Orchestra.
In my view, the purity of note but resulting "acidity" of the woodwind sound is what keeps it as the solo section of an orchestra, and leaves the string section to be the orchestra's backbone. There are of course, wind bands, which are effectively orchestras without string section. These are popular, particularly in the USA, where a lot of very good music has been written specifically for this combination of instruments. Here, the woodwind fulfil the position of the string section of a conventional Symphony Orchestra, but my ears sound is very different, being more insistent, harsher, acidic. This isn't to decry this instrumental lineup, because the music can be truly marvellous, but my ears it does lack the subtlety of colour that is achieved by the string section of the traditional Symphony Orchestra.
Another way of looking at it, is that the brass and woodwind instruments are used in a jazz band (I should say, certain of the brass and woodwind instruments are used in the jazz band). It is far less common to have a stringed instrument, or indeed a complete string section, in a jazz band or jazz orchestra. When a jazz orchestra does include a string section, it changes the character of the sound to a much more expressive and less demanding ensemble. Perhaps demanding is the wrong word, but meaning is 'in your face", lively, (really I don't know how to describe it), but a jazz band is about dance music, and in the early jazz bands was about religious exuberance, so in your face brass and woodwind instruments are very appropriate here. So, as always, is horses for courses, and whether you're talking about a Symphony Orchestra, a string quartet, jazz band or ceilidh band, the instruments become popular in those various lineups are those instruments that enable the music to be conveyed in the best possible way.
Yet another reason why a string section is often considered as the backbone of the orchestra, is because of their numbers. The string section may have 12 first violins, 12 2nd violin, 10 violas, 10 cellos, 5 double basses. The flute will normally have 2 flutes, occasionally 3, and from time to time the tiny piccolo, and even less frequently the base flute. So, as an orchestra is a very visual thing, and the visual impact of the orchestra at a concert is a big part of what makes a live orchestral performance so different to listening to a CD, the physical backbone of the orchestra is definitely the string section. This is though, only a matter of volume. Piccolo, because of his very high frequency and purity of note (limited harmonics) can scream its way through a full orchestra playing Forte. A violin doesn't stand a chance. Thus they have to gang up in large numbers. It also has the added advantage that, because a violin or any other stringed instrument for that matter, is a wooden box with a stick, strings made of stretchy stuff that tends to change its characteristics with temperature and humidity, as does the wood, with no fixed places machined into the instrument for definite notes, (a flute has holes drilled in it to high accuracy, with a mechanism for covering those holes, so flautist more or less – though not quite, I'm going to get myself into trouble from flautist here – blowing the whole, plumper finger on the key, and outcomes write notes. More extreme still is the piano. Every note has its own string, or set of strings. The string or strings will only ever play that one note. A piano tuner will have spent an hour or 2 getting all the strings on the piano tensioned up to be perfectly in tune. The pianist hits a key, and the correct load comes out (assuming they had the right key of course!) A violin is quite different. Every fraction of the 2nd, the violence is playing a new note, and there is absolutely no guidance where their finger should be positioned on the string to achieve that note. There hasn't been a machine shop working on the instrument to make sure that when they put their first finger down it plays a correct note. Some tuner hasn't spent hours on each note of the instrument making sure that when the vileness does something rather, exactly the right note comes out. Each note played by vileness does a pure guess, almost pure guess. By dint of practice, they can get their finger in more or less the right position on a lot of notes, by the feel of where their finger on hand is. With the position sensors that feedback to our brain the positions of hands arms fingers et cetera, are highly inaccurate. When we pick up a cup, the position sensors of our limbs are not accurate enough to do that without knocking the cup over. We rely on vision and touch sensing information to add to our very approximate position sensing information, to enable us to pick up a cup quickly and accurately without spilling it. The violinist has to rely on hearing to provide that extra bit of information to enable them to play their instruments in tune. As a result, a violinist is always playing the wrong note and then hopefully, correcting it quickly, before anybody notices that is wrong. All sorts of tricks are incorporated into playing techniques to make this happen more effectively, for example the slide into position when moving from a low note to a high note, where the high note requires a much more accurate figure position in the low note, and could not be hit accurately every time without the slide (the slide enables the player to tell by his hearing when the correct note is being approached, and bring the slide to a standstill in exactly the right place.
You're probably thinking that I've digressed rather, the probably I haven't, because what I was getting to is that in a string section you need the numbers of instruments to make it sound in tune. In a string quartet, you have 4 instrument each playing their own line of the tune. They have to play very well in tune to make this harmony sound correct, but if they were to play in unison, then the accuracy of intimation would have to be very much higher to make it sound okay. Thus, very few passages in the string quartet have any of the instruments playing in unison. If they do, then normally it is to have the instruments playing an octave or more apart, and here one of the instruments (normally the upper part), plays more quietly so that you don't get a clash of sound because of the slight imperfections in intimation. Is the same effect that makes an Arabic orchestra, where whole sections of the orchestra are playing in unison, sound edgy and acidic. It is this same thing that is used in the musette tuning of an accordion, where 2 or 3 reeds that play the same note are tuned ever so slightly different, to give a harsh penetrating and acidic sound.
So, in a symphony orchestra with perhaps a dozen first violins, all playing ostensibly the same notes in perfect unison, the errors in pitch are averaged out, and the whole section will sound silky smooth. Bring it down to 2 violins in the first section, and it would sound very harsh and acidic. So not only are the large numbers of strings essential because of the limited volume of each instrument, but it's essential to get the silky smooth sound that is characteristic of a larger string section. Bandleaders like Mantovani use this fact in his orchestras, where the silky smooth Mantovani sound was achieved in part by having a very large string section (and in part by introducing artificial reverberation by scoring the music so that instruments played slightly apart from each other in timing. All very clever stuff.)
Things have been different in different periods of history. In Handel's day, it was common to have as many as 8 oboes playing in unison, and what it might have sounded like is inconceivable to modern ears. (I've got some idea what it probably sounded like, because whilst in Spain some 7 or 8 years ago my wife and I stumbled across a festival. It was in the paved area in front of one of their great cathedrals, and there was a band or orchestra playing for communal dancing. It is very unusual, as people were coming home from work and joining in the dancing on the way, dropping their briefcases and suit jackets in a pile next to each dance group. Anyway the significant thing was that the orchestra included a section comprising some 15 oboes. They weren't quite the same as modern orchestral oboes, probably being a baroque form of the instrument. What was clear though was that they were going to win the day, whatever anybody else wanted to play or listen to. Raucous, harsh, insistent, cutting, I don't know what the correct word is, but it was probably some word that combines all these qualities.
Despite all these variations, including the Chamber Symphony of Schoenberg, which was written for 15 solo instruments, the large string section of an orchestra is standard now.