Haecceities are non-qualitative properties for individuation but the current theories about haecceities are still to be much more explored. This paper aims to develop a “haecceity mereology” – that is, an ontological system that understands substances as mereological combinations of haecceities and qualitative properties. In this way, the view developed is an alternative to Paul’s : 578–96; 2006. “Coincidence as Overlap.” Noûs 40 : 623–59) mereological approach. Three rules are proposed: If S is a substance, then there is one and (...) only one haecceity which is S’s qualitative part; For all the fusions with the same haecceity, at most only one of those fusions is a substance; When all the relevant elements are abundant, every element must overlap at least one substance. This is the first ontology of haecceities in the recent literature and would be a model for systematic metaphysics. (shrink)