The human eye has various axes and angles, of which the angle Kappa is an important indicator of the centrality of the human eye and is widely used in ophthalmic surgery. Proper preoperative evaluation and application of the angle Kappa facilitated the achievement of optimal postoperative visual quality. Chord mu is a new term that has emerged recently to better express the angle Kappa. The two concepts are not well understood clinically, limiting their usefulness. Therefore, to better understand the angle Kappa and chord mu, the definitions and connections between them are presented separately in this paper. Meanwhile, the application of angle Kappa in strabismus surgery was summarized, the method for compensating large angle Kappa in corneal refractive surgery and the clinical significance of angle Kappa in predicting postoperative centrality of multifocal intraocular lens(MIOL)in phacoemulsification combined with MIOL implantation were discussed, with a view to providing references for clinical work.