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The most historic ensemble, one of the most widely used and certainly the most diverse, the orchestra is a grand group of instruments that has evolved massively over centuries. It began as a small ensemble and has grown enormously over time, and numbers and options are endless, ranging from as little as approximately 40 players to - well, unlimited amounts - Mahler's 8th is coined "Symphony of a Thousand" for a reason. It can actually be quite an intimidating force - I know in the past I have found it quite overwhelming, with 19th/20th century composers such as Beethoven, Strauss, Elgar and Stravinsky utilising the ensemble to awesome effect. But it can be broken down, and here I give a little crash course into the epic, enigmatic musical group.
The orchestra is basically split into four sections: brass, percussions, strings and woodwind, and auxiliary instruments may be added (such as a harp, piano, although these often fall into one of the above categories - strings and percussion respectively). Brass fundamentally consists of horns, trumpets, trombones and tubas; strings: violins, violas, cellos and double basses; woodwind: flutes, oboes, clarinets and bassoons; percussion is viewed as far more optional, but I will return to this. It's all about balance, really, and it entirely depends on your music. Many composers since the 19th century have authoritatively tweaked numbers to suit their needs, and while numbers generally grew from 1830 until the early 20th century, the numbers are constantly changing. So I'll give you a somewhat standard example: 2 flutes (1 doubling piccolo - in that 1 flautist may switch to a piccolo (but they must be given ample time to switch)), 1 oboe, 1 cor anglais (a tenor version of the oboe), 2 clarinets, 1 bassoon and 1 contrabassoon (an even lower version of the bassoon); 6 horns; 3 trumpets, 3 trombones, 1 bass trombone, 1 tuba; timpani, bass drum, cymbal; 2 harps; 2 violin sections, 1 viola section, 1 cello section, 1 double bass section. You may have noticed that I separated the horns from the rest of the brass, and my reasons are that horns are often treated as members of the woodwind section, and further are referred to as "wind" instruments. Percussion is easily the most diverse section, with a myriad of standard to exotic instruments at a composer's disposal: timpani, bass drum, cymbal, gong, triangle, woodblock, xylophone, marimba, waterphone - the list just goes on. And composers are always welcome to spice up the orchestra with many other instruments too, but most common non-percussive 'auxiliary' instruments include the piano, harp, saxophone and euphonium. Now that's a lot to take in, but in summary, there is a lot of room to manoeuvre despite being in place a traditional skeleton.
The fifth 'section' of the orchestra consists of one man, and is traditionally most certainly mandatory: the conductor. Although conducting in one way or another began in the 16th and 17th centuries, it truly flourished with the input of musicians such as Wagner, Berlioz, Spohr and most importantly, Mendelssohn. The conductor's main roles are to orchestrate the rehearsals (although this was not always case), and this is hugely superior to anything 'on the night', to beat time during performance, to enforce and appear as an authoritative figure, and to encourage emotions and performing styles. The conductor is the leader. The performance becomes his (or her) performance, whereby his name will appear on the recording sleeve notes, rather than the performers - and he has quite a big job, to be fair. Every performance is different, and different conductors can create vastly different versions of a certain piece as each conductor interprets the pieces differently. That said, should a specific interpretation turn the piece upside down or change it completely, the said conductor might find his or her reputation rapidly dwindling.
Without delving too far into compositional methods, it is important to review how exactly instruments are used. On the surface, instruments can either be used solo or in combination, and when all instruments play, this is known as 'tutti', an Italian word meaning 'all' or 'together'. The orchestra is specifically designed so that there is a bass, middle and treble, or high-end, like most other ensembles, and as soon as you start tweaking numbers, there is potential to spoil the balance. While you can use your double basses, tubas and bassoons to provide bass parts and violins and flutes to provide a melody, this is basic, albeit used very often; one thing I can't stress enough with the orchestra is that possibilities are endless - feel liberated when writing. If you want your cellos to play at the top of their register while the trumpets are playing at the bottom, then go right ahead; if you want the melody in the bass instruments, then do it! The repertoire provided by renowned composers, particularly those post-Beethoven (including his contemporaries) like Wagner, Mahler and Strauss, offer shining examples of how to exploit the instruments of the orchestra; but they've collectively far from exhausted the options. Some orchestral techniques include divisi, whereby a section of instruments, such as the violas, are split into multiple sections (usually not exceeding 3), solo, where any instrument can play alone, whether that be a flute, one of your violins from one of your violin sections, or a timpani, and tutti, as previously mentioned. You need to decide, when writing, how many instruments you want playing when. Create moods, build excitement and keep the orchestra on their feet; or keep things the same like Ravel did with 'Bolero' - it's not wrong, and it's certainly still enjoyable. You could have all of your higher instruments playing in unison; you could have only your brass section playing a fanfare. Worth noting is that certain instruments and sections do have popular roles, though they are by no means obligatory. For example, you may never give the melody to the violins in a piece, although they are one of the most common instruments to carry one. A reputable technique is to fluently employ the woodwinds throughout a piece, as amateur composers may only bring them in to highlight a section - equally, the world (or orchestra, rather) is your oyster, so do whatever you want.
I've made reference to a few composers in this discussion, particularly those of the 19th and 20th centuries; it is worth collating these, whilst not excluding pre-19th century composers who wrote for orchestra, such as Bach, Lully, Scarlatti, Haydn and Mozart. 19th/20th century composers, however, include Schumann (a good example of a composer who perhaps had orchestral-balancing troubles), Berlioz, Lizst, Brahms, Wagner, Mendelssohn, Tchaikovsky, Mussorgsky, Debussy, Elgar, Ravel (a top-class orchestrator), Strauss (ditto), Mahler (also ditto, but so much more - he was essentially the last developer of the symphony), Stravinsky, Prokofiev, Bartok, Vaughn-Williams, Britten, Shostakovich, Schoenberg, Bernstein, Ives, Cage and Glass - and I emphasise the word include, as I have inevitable missed out some key composers from this list. One I have deliberately left out, however, is Beethoven, as he deserves a section for himself...
Ludwig van Beethoven is one of the most known names in Classical music, perhaps only rivalled by Mozart. He was an eccentric man; a genius, and a revolutionist. It is commonly believed that with his third symphony, 'Eroica', originally dedicated to Napoleon but now only to a once-"great man", that the Romantic period began - his composition and use of the orchestra was groundbreaking and rather controversial. But he inspired a generation, and continues to inspire to this day. 'Eroica' is one my favourites, and I would strongly urge you to give it a listen. He was not a melody-writer, per se - that was left to the likes of Schubert - but his overall compositions are unsurprisingly fantastic and it's his revolutionary outlook that causes him to be respected and his repertoire revisited to this day. And it's thanks to him, in some cases greater than others, that the above listed 19th/20th century composers came to be and wrote in the style that they did - and ultimately, utilised the orchestra in the way that they did, to the point where John Cage would stand on the conductor's podium with hands poised for 4 minutes and 33 seconds, with the orchestra sitting in complete silence (see '4.33', or just stay silent for that length a time - your call).
Although there is a diverse world in what we call Modernism (our present day Classical music), the orchestra can be heard most commonly these days in film music. From the late 19th century with films being scored, most notably Soviet Union productions, film music has always, at least somewhere, called for an orchestra. The golden age of Hollywood saw composers such as Bernard Herrmann, Max Steiner and Erich Korngold; later years saw composers like Jerry Goldsmith, John Williams and Alan Silvestri - and all of the above are heavily influenced by 19th/20th century output - certain Wagnerian music could easily be mistaken for John Williams. More recently, composers such as Alexandre Desplat and David Arnold have maintained the well-rounded orchestra, whereas composers such as Hans Zimmer have broken it down and made his instrumentation more selective, although Zimmer, amongst others, rely heavily on electronica. In a world that sees 'pop' (or 'popular' music) as a range of dance music and Justin Bieber, film music carries through the legacy of the orchestra.
I'm currently in my third year studying BA Music at University of Leeds, and I owe a lot of my knowledge on this subject to what I have learnt there, although a large portion of it is down to own research inspired by what I have learnt. Thank you for being patient and taking the time to read this - I enjoy sharing my interests and Classical (encompassing 19th/20th century music) music is progressively becoming one of my main ones. I strive to write music for film/TV, and I strongly believe studying these 19th/20th century composers and how they write music and utilise the orchestra is paramount in gaining a broader understanding. In the meantime, I hope you have learnt a little bit about the orchestra yourself.