How similar are two melodies? Proposed in 1990, the Mongeau-Sankoff algorithm computes the best alignment between two melodies with insertion, deletion, substitution , fragmentation, and consolidation operations. This popular algorithm is sometimes misunderstood. Indeed, computing the best edit distance, which is the best chain of operations, is a more elaborated problem. Our objective is to clarify the usage of the Mongeau-Sankoff algorithm. In particular, we observe that an alignment is a restricted case of edition. This is especially the case when some edit operations overlap, e.g. when one further changes one or several notes resulting of a fragmentation or a consolidation. We propose recommendations for people wanting to use or extend this algorithm, and discuss the design of combined or extended operations, with specific costs.