Requirements documents often describe the system on different abstraction levels. This results in the fact that the same issues may be described in different documents and with different vocabulary. For analysts who are new to the application domain, this poses a major orientation problem, as they cannot link different concepts or documents with each other. In the presented paper, we propose an approach to map concepts extracted from different documents to each other. This, in turn, allows us to find related passages in different documents, even though the documents represent different levels of abstraction. Practical applicability of the approach was proven in a case study with real-world requirements documents.