This educational talk will explain why we need a vertical reference for our coordinates, how we define "up" and "height". How elevations were measured in the past, and how we now use GNSS to do it, and the implications of that. What is "the geoid" (gravitational model of the earth) and its differences with respect to the ellipsoid. Different types of heights (orthometric, normal, dynamic) and how we use different geoid models. Finally I will talk about how PROJ.org (open-source library) is supporting vertical coordinate reference systems with the grid files available in PROJ-data (open-data).