Background: Kaplan’s Cardinal Line of the hand was first described in 1953 by Emmanuel B. Kaplan as a line “drawn from the apex of the inter-digital fold between the thumb and the index finger toward the ulnar side of the hand, parallel with the middle crease of the hand”. It was an anatomic description provided in the text “functional anatomy of the hand” and was intended to serve as an approximate location of the deep structures of the hand which a surgeon would aim to avoid, as well as provide a margin of safety to the superficial palmar arch (SPA) which lies distal to it. Over time, its description has evolved with textbooks littered with slight variations in its exact location. Methods: We performed a literature review to assess the most accurate description of Kaplan’s Cardinal Line with reference to its reliability to avoid relevant structures in hand surgery. We review the literature of cadaveric studies attempting to identify the most accurate line and explore reasons that Kaplan’s original definition was modified. Discussion: Several cadaveric studies find the original definition of the line to be reliably proximal to the SPA. However, comparisons of the 4 dominant definitions of the line have found Kaplan’s original to be less accurate in locating deep structures of the hand. Modern descriptions may attempt to improve the accuracy of the line by providing constant anatomical landmarks (e.g. bony prominences), thus reducing patient variability.