Two textbase indexing methods enjoying wide applicability are the inverted index and the Superimposed Coding based Signature File (SC-SF). The former is most efficient in query processing, whereas the latter excels in storage utilization. Building on previous results, we propose a new hybrid structure (S-Index) which has a tunable performance. At the one extreme end, S-Index turns into a signature file with zero information loss, so that queries are processed faster than in ordinary SC-SF. At the other extreme end, S-Index turns into an inverted index. The advantage of the proposed access method is that the textbase index may now be tailored to the query profiles of user classes: for frequently queried textbase sections S-Index performs like an inverted index, whereas the bulk of the textbase is indexed in the form of a signature file. The S-Index structure is presented in detail, together with performance analysis results.