This paper proposes a model called mapping & correlation (Macor) to represent nested schema mappings. With Macor, a full mapping is modeled as a number of simple atomic ones that are correlated with correlations. The expressive power of Macor by a fragment of XQuery called CoXQ is studied and the issues in implementing Macor are addressed. Preliminary experimental results show that with Macor a full mapping can be modeled incrementally in a piecemeal fashion, and in refining or maintaining the mappings, Macor makes it possible to locate modifications to few atomic mappings and correlations, and reuse other parts of the full mapping.