Natural light and Gothic architecture are two inseparable concepts. The reality of Gothic cathedrals cannot be understood without comprehending its intrinsic relationship with the symbolic light of their spaces. Theories about Gothic light and its symbolic and theological value are abundant; yet, there is no study focusing on the fact that light as a physical reality can be quantified, qualified, and therefore, classified. This article explores the direct relationship between cathedral shape and volume and luminous result through the study of some unpublished factors and qualities found along the in-depth analysis of six unique Gothic spaces. This relationship between shape and light aims to become a new conditioning factor to be taken into account in the future, helping to understand and respect the main engine of the Gothic architectural project at its origins: light.