Visual languages can denote programs directly as flow graphs, enhancing transparency by increasing the correspondence between visible and underlying program structures. Communication among concurrent processes can be depicted by drawing links, with internal structure refined at lower levels. This encourages safe programming practice, since processes can talk only if they are explicitly connected at the level of invocation. Such a linking of interfaces can be understood as the use of a time stream parameter connecting two or more relations. The structures available in this approach can be used to construct asynchronous and synchronous communication links. Because the semantics in a relational calculus, it is mathematically tractable; because the syntax is visual, it removes some of the notational inconveniences that have hindered attempts to popularise this style of programming. It therefore seems to have some advantages over other models of concurrency such as CCS, CSP and Actors.