An alloy of copper with 3.5-10% tin, with the addition of up to 1% phosphorus. The phosphorus in the blend makes the metal more fluid when molten, and easier to cast. It is also said to improve the mechanical properties of the alloy.
Phosphor bronze is used in many applications (bolts, springs, dental bridges, ship propellers, cryogenics, and more); in the music world, it is commonly used for acoustic guitar strings (D’addario takes credit for making the first phosphor bronze strings), where it is valued for its resistance to corrosion and oxidation and resistance to fatigue — meaning it can have a longer life when used as a string than some other alloys.
Phosphor bronze is also sometimes used to make instruments such as saxophones. Instruments made from this alloy tend to have a reddish-orange color, as opposed to the more-standard gold of lacquered brass.