In this article, I analyse how conventional height-specifier gestures used by speakers of Yucatec Maya become incorporated into Yucatec Maya Sign Languages (YMSLs). Combining video-data from elicitation, narratives, conversations and interviews collected from YMSL signers from four communities as well as from hearing nonsigners from another Yucatec Maya village, I compare form, meaning and distribution of height-specifiers in gesture and sign. Co-speech gestures that depict the height of upright entities - performed with a flat hand, palm facing downwards - come to serve various linguistic functions in YMSLs: a noun for human referents, a verb GROW, a spatial referential device, and an element of name signs. Special attention is paid to how height-specifier gestures fulfil a grammatical purpose as noun-classifiers for human referents in YMSLs. My study demonstrates processes of lexicalisation and grammaticalisation from gesture to sign and discusses the impact of gesture on the emergence of shared sign languages.