Defining a layout of diagram elements. A method includes receiving user input. The user input includes one or more declarative statements specifying conditional patterns based on attributes of diagram elements. The conditional patterns define layouts of diagram elements. Implementation of the layouts is dependent on conditions defined in the declarative statements and one or more values of one or more of the attributes. The method further includes organizing the conditional patterns as a pattern definition. The pattern definition is stored on a computer readable medium. The pattern definition is stored such that the pattern definition is retrievable by an application program that uses the pattern definition to evaluate the conditional patterns using values of attributes of one or more diagram elements. The application is also configured to display representations of the diagram elements according to the layouts when conditions for implementing the layouts are satisfied.