In a semantic environment data is described by ontologies and heterogeneity problems have to be solved at the ontological level. This means that alignments between ontologies have to be created, most probably during design-time, and used in various run-time processes. Such alignments describe a set of mappings between the source and target ontologies, where the mappings show how instance data from one ontology can be expressed in terms of another ontology. In this paper we propose a formal model for mapping creation. Starting from this model we explore how such a model maps onto a design-time graphical tool that can be used in creating alignments between ontologies. In the other direction, we investigate how such a model helps in expressing the mappings in a logical language, based on the semantic relationships identified using the graphical tool.