NEUMA is an on-line library that stores collections of symbolic scores and proposes a public interface to search for melodic pieces based on several kinds of patterns: pitches-based, with or without rhythms, transposed or not. In addition, searches can be either exact or approximate. We describe an index structure apt at supporting all these searches in a consistent setting. Its distinctive feature is an encoding of the various information that might be involved in the pattern-matching process with algebraic signatures. The properties of these signatures are suitable to represent in a compact and expressive way the sequences of complex features that constitute a melodic description.