—This paper presents results of an empirical research study of the Unified Modeling Language (UML) use in practice. We employed a selective range of research methodologies including in-depth semi structured interviews and quantitative analysis of > 3500 UML diagrams related to open source projects in GitHub. The aim of the study is to provide greater understanding about the use of UML and to particularly shed light on the use of the visual variables (i.e., color, size, brightness, texture/grain, shape and orientation) in practice. The theoretical perspective of the study is to explore the usefulness of the visual variables in UML. These latter are highly significant in reducing the cognitive load of human beings, when effectively employed. As with all qualitative study, findings should be carefully interpreted, they should be seen as providing better understanding about the aforementioned scopes. We conclude by discussions of the obtained results and lessons learned for future researches.