(from the Latin justus — correct), the set of operations that bring measuring instruments into a state ensuring their correct functioning. Alignment removes the errors revealed when measuring instruments are inspected or verified. The main alignment operations are: checking the state of the measuring instrument; regulating the relative position of parts and assemblies with screws, shims and so on; removing defects by grinding, lapping and finishing; and replacing individual parts and assemblies. The alignment of optical systems consists mainly in regulating the relative position of the optical parts (lenses, prisms, mirrors) in order to centre them and to secure a high quality of image. Optical systems usually include devices that regulate the position of the optical parts and clamp them during alignment. The term “alignment” is usually applied to measuring instruments, in particular opto-mechanical ones; for mechanisms the term “adjustment” is more often used.