Variability models represent the common and variable features of products in a product line. Several variability modeling languages have been proposed in academia and industry; however, little is known about the practical use of such languages. We study and compare the constructs, semantics, usage and tools of two variability modeling languages, Kconfig and CDL. We provide empirical evidence for the real-world use of the concepts known from variability modeling research. Since variability models provide basis for automated tools (feature dependency checkers and product configurators), we believe that our findings will be of interest to variability modeling language and tool designers. Categories and Subject Descriptors D.2.2 [Software Engineering]: Design Tools and Techniques; D.2.8 [Software Engineering]: Metrics General Terms Design, Languages, Measurement Keywords Configuration, empirical software engineering, feature models, product line architectures, variability modeling