brass
Brass is the bright, gold-toned metal of trumpet bells, door handles, plumbing fittings, and old keys. Chemically it is simple: copper plus zinc. That one addition makes copper stronger and cheaper (zinc is less costly than copper), gives it a warm yellow color, and makes it wonderfully easy to machine and shape, which is why brass has been a favorite for decorative and precision hardware for centuries.
The amount of zinc sets the properties. Up to about 35 percent zinc dissolves fully into copper's face-centered-cubic lattice as a single-phase alpha brass: soft, ductile, and easy to cold-form into cartridge cases and deep-drawn parts (70/30 cartridge brass is the classic). Push past about 38 percent zinc and a second, harder phase (beta) appears, giving alpha-beta brasses that are stronger and hot-work and machine beautifully but are less ductile cold (60/40 Muntz metal and free-machining brasses). Zinc strengthens copper by solid-solution strengthening: the differently-sized zinc atoms distort the lattice and make dislocations harder to move.
Brass is chosen for looks, machinability, acoustic quality, and moderate corrosion resistance in a huge range of fittings, instruments, and fasteners. Its honest weakness is a specific one: in the wrong water or atmosphere, high-zinc brasses can suffer dezincification (the zinc leaches out, leaving weak spongy copper) or stress-corrosion cracking (season cracking). Adding a trace of tin (admiralty brass) or arsenic holds this off. And do not confuse it with bronze: brass is copper-zinc, bronze is copper-tin.
70/30 cartridge brass (70 percent copper, 30 percent zinc) is single-phase and so ductile it can be deep-drawn into a seamless bullet casing in one press stroke; 60/40 Muntz metal, with its harder beta phase, is stronger but must be worked hot.
More zinc means stronger and cheaper but less cold-ductile, and eventually a second (beta) phase appears.
Brass is copper plus zinc; bronze is copper plus tin. The names are often swapped in everyday speech, but to a metallurgist they are different alloy families with different properties and uses.