I was asked about this one by my lovely friend Linda, a good friend and a super editor, too. It’s sometimes hard to think up new troublesome pairs to write about, so I love it when people suggest them to me, often because they’ve encountered someone else confusing them, sometimes for themselves (it’s the former in Linda’s case).
To breach something (the verb) means to make a hole in it OR to break something like an agreement or a law, or simply a code of conduct. A breach (the noun) is the result of someone breaching something: it’s the act of breaking an agreement or a law (a breach of conduct) or a gap or hole in a wall or other barrier. “By wearing jeans in the dining room, he breached the club’s formal dress policy”. It’s often used in a military sense, but in a general one, too.
The breech (noun only) is the part of a cannon, gun or rifle barrel that is behind the bore. The old-fashioned use of the word means a person’s buttocks, but this survives chiefly in the term “a breech birth” which occurs when a baby is turned around in the womb so that its bottom or feet emerge first.
Breeches (which that last sense of breech comes from) are short trousers, ending just below the knee, which are nowadays used in ceremonial or riding dress.
Breaches are multiple gaps in a wall, etc. or multiple infringements of policy and laws.