The crossbow
A crossbow mounts a short, powerful bow horizontally on a stock and holds the string on a latch, firing a short arrow called a bolt.
Bolt, not arrow
A crossbow shoots a bolt, a short, stiff arrow typically 16 to 22 inches long depending on the bow. Because the bolt is launched off a rail rather than flexing around a riser, crossbow spine and length are set by the machine, not by the shooter's draw length, so each crossbow has one correct bolt.
That is why the crossbow side of fitment is model-by-model: the maker specifies the exact length, the nock shape and a minimum total weight for each bow, and those numbers are not interchangeable between brands.
The nock and the dry-fire trap
The nock, the fitting at the back of the bolt that engages the string, is the single most important compatibility point on a crossbow. Some bows need a captured or clip-on nock so the string cannot slip off the bolt at speed; others use an indexed half-moon nock that must be seated square to the string.
Fitting the wrong nock, or a half-moon nock a quarter-turn off, can let the string pass over or under the bolt and dry-fire the bow. A dry-fire dumps the full stored energy into the limbs and can destroy the bow and injure the shooter, which is why the fitment tool leads every crossbow page with it.
Minimum weight and speed
Every crossbow has a minimum total arrow weight, commonly framed as at least five grains per pound of draw weight and never below the maker's stated figure. An arrow that is too light behaves like a dry-fire even though a bolt is on the rail.
Chasing speed with an ultralight bolt is the classic way to crack a limb or a cam. The published speed of a crossbow is only true with the arrow it was rated with, so weight and speed are read together.
Find the exact bolt for your crossbow
Open the BoltFitFinder fitment toolLength, nock and minimum weight by modelFind crossbow bolts carbon on eBayaffiliateRelated
Sources: Wikipedia: Crossbow ยท Crossbow nock types (Best Crossbow Source)