Cannon, with two n’s, is a long heavy gun on wheels. Lose an n and the word canon is a set of rules or traditions. If you thought these words were spelled the same, you’re not wrong: they used to be.
In the 1800s, cannon got back to its Latin roots, canna, meaning "reed or tube," and doubled the n. A cannon is basically a giant tube that shoots projectiles. You might even see a tee-shirt cannon blasting shirts into the crowd at a large sporting event. If you’re a loose cannon, you’re wild and unpredictable. Here are some examples of cannon:
Like at my fifth-grade school, when we went to the Saratoga battlefield, I had this accident with a Revolutionary War cannon. (The Lightning Thief)
They must think Hudson is a loose cannon and I’m the summer school know-it-all. (What If It’s Us)
The word canon came from the church, meaning "church law." It branched out to mean a "catalog of approved authors" in the 1800s. Now the canon refers to a complete, accepted body of work or related books. Canon also refers to original content — if you’re writing Harry Potter fan fiction, the source material is canon, but not the fan fiction that you write. Check out canon in the wild:
From Cicero onward, it has been conventional to divide up the business of oratory into five basic parts — sometimes known as the "canons of rhetoric." (Words Like Loaded Pistols)
The canon of black autobiography sensibly includes scores of books about resistance to the American system. (The Other Wes Moore: One Name, Two Fates)
A tip to remember the difference: a cannon has an extra n as ammunition. Canon is the original.