In principle, anything that generates sound is an instrument.
You probably always thought a barking dog was a nuisance. Now you can regard it fondly as a musical instrument.
The definition of an instrument as a sound generator is a very broad definition and this section provides more precision. It provides an overview of the different types of instrument available for writing music, the range of frequencies they produce, and the process of combining instruments together, instrumentation.
Writing music for different instruments is an excellent way to develop your skills. A wide variety of instruments are demonstrated throughout the guide so that you can hear the impact that instrumentation has on timbre.