Picture a sickle bar sliding back and forth across a row of fixed guards. For a clean cut, two things must be right: where the knife section sits relative to the guard at the end of a stroke (registration), and whether the guards themselves form a straight, even line (alignment).
Both checks come from the maintenance manual of the mower, since a mower depends on a fast reciprocating knife shearing cleanly against guards to cut standing forage without dragging or missing stems. Reapers and binders use a similar idea but the terms registration and alignment are specifically drawn from mower cutter bar servicing.
\[\boxed{\text{Mower}}\]