"Nock is a family of computational languages derived from the ski combinator calculus. It serves as the isa specification layer for the Urbit and NockApp systems. This article outlines the extant historical versions of the Nock combinator calculus and reconstructs the motivation for the changes made at each kelvin decrement. It begins with an exposition of Nock as a tool of computation, outlines the history of Nock’s decrements, and speculates on motivations for possible future developments."