The explicit definition of variability in software product lines is a key difference between the devel-opment of single software systems and software product line engineering. More and more companies maintain several software product lines which focus on different types of products, market segments, and/or domains. Those product lines typically share commonalities and variability. The companies thus face the problem of managing communality and vari-ability across different product lines. In this paper we identify essential requirements for the documentation of requirements variability across product lines. We propose a meta model for structur-ing the variability information, sketch a prototypical realisation for managing variability across product lines in DOORS, and illustrate the use of the meta model in a small example. We further report on ex-periences made with the proposed variability model-ling approach.