This paper introduces the T-compound, an agent interaction pattern we propose as reusable architecture in the design of multi-agent systems. We explain why this model and its underlying concept of over-hearing are relevant and specific to agents. Then, we describe two strategies of implementation, together with their qualities and weaknesses. We then identify the T-compound in various agent-based projects to confirm its properties of reusability and composability that turn it into design pattern. The paper ends with reports of other implementations that address overhearing and related concepts.