Mixed-member proportional systems (MMP) are a family of electoral systems which combine district-based elections with a proportional seat allocation. Positive vote transfer systems belong to this family. This article explains why they might be better than their siblings, and examines under which conditions full proportionality is reached. A formal model shows that full proportionality depends on the number of compensation mandates available, and on the degree of coordination among voters and parties. The model is applied to six elections in Hungary.