Message Sequence Charts are a popular formalism for the design of distributed systems executions based on pomset composition. However, this formalism in its basic form is not expressive enough to model typical behaviors such as sliding windows executions. A solution is to embed in MSCs the expressive power of communicating automata, as in CMSCs (another extension of Message sequence charts). However, most basic problems become undecidable for CMSCs. This paper proposes an extension to message sequence charts which extends the expressive power of MSCs while preserving the decidability of some properties. This modification extends sequential composition, but still rely on compositions of closed communication patterns (messages are emitted and received in the same pomset). This paper gives a definition of this new formalism called ``sliding'' message Sequence charts (or SMSCs). SMSCs can be ill-formed, and we provide a decision procedure to detect and transform such SMSCs inti well-formed ones. Then, the expressive power of SMSCs is compared to that of HMSCS, CMSCs, sure CMSCs and HMSC projections.