A signal instrument is a musical instrument which is not only used for music as such, but also fit to give sound signals as a form of auditive communication, usually in the open air. Signal instruments are often contrasted with melodic and diatonic or chromatic instruments ("a musical (rather than signal) instrument"[1] is not uncommon phrasing). To make the message audible at a distance, percussion and brass instruments, which are generally loud, are chiefly used for this purpose. There are contemporary instruments which evolved from signal instruments, such as the natural horn evolving to the trumpet.
Before the introduction of modern technological communication, signaling over a distance was often a very good way to pass messages, especially in difficult terrain such as the mountains (e.g. Alpine horn, equivalents are still used in parts of the Himalaya) or sparsely populated plains or forests (the tam-tam type of drums as with American Indians and jungle drums), sometimes using rather elaborate code systems to pass even complex information.
Another ancient function, which has survived into modern urban life is to assemble or warn a whole population or congregation at large, usually not coded or just for a few common cases, as with a conch or bells in a church or belfry: a variation for more local use is the gong.
Many types, especially the older ones, have also survived for ceremonial use, as in religion (often conservative in its forms) or gong ceremony.
While the types mentioned above are mainly used from one spot, signal instruments may also be useful to communicate on the move, when many alternatives were less practical. Naturally then instruments are preferred which are not too delicate to be moved, and often not too heavy (except when the musician is mounted or in a vehicle) to transport, or even better be played during march or even chase. Thus hunters traditionally use a hunting-horn to communicate, while drums (often just improvised 'percussion') were rather by drivers while shouting. When the social elite practiced hunting as a prestigious outdoors social activity, music was often present at any stage, e.g. the court of Versailles had composers such as the Danican family and Philidor write special symphonies for a court orchestra to accompany the royal hunt party, prominently featuring percussion and winds but also including non-signal instruments, even strings.
In the (para)military and similar, mainly uniformed, corps such as police, the tradition of march music stems from the use of signal instruments, mainly metal winds (as the modern bugle and clarion[1]), originally (and still) to pass standardized orders (often at a small tactical level), while percussion and flutes served mainly to march on the beat; especially when a larger force is gathered with ceremony (just drums often do, as in ruffles and flourishes or accompanying formal administration of corporal punishment), both can be combined into a band.