Acoustic guitar strings are commonly made of bronze, phosphor bronze, brass, nickel, silk, and steel.
Each material has its own timbre. Bronze sounds bright, with bell-like clarity and a wide treble-forward frequency response. As the name suggests, phosphor bronze sees phosphor added to the alloy to slow oxidation and extend string life. Phosphor bronze strings tend to sound a little darker and warmer than bronze.
Brass strings have plenty of top-end jangle, and nickel strings have a warmer tone making them an excellent vintage choice, as does monel – a nickel-based alloy that many guitarists swear by for getting the best out of their tonewood.