Overhearing is an indirect interaction type that enacts agents to listen to direct interactions among other agents without taking explicit part in the exchanges. In this paper, we propose a formal model of overhearing named T-compound and a methodology to describe generalised interaction networks in Multi-Agent Systems. The compound is defined with the π-calculus as an interaction composite. It is handled as an interaction primitive distinct from the traditional point-to-point one, so that our methodology can treat both cases homogeneously.