► Object: The interaction between objects takes place in a system. An object is depicted by a rectangle with the name of the object, preceded by a colon and underline.

► Relation/Association: Association among objects is linked by connecting them. The cardinality can be depicted by placing qualifiers on either ends.

► Messages: An arrow that commencing from one object to the destination object. This depicts the interaction between objects. The sequence or order of the interaction is depicted by the number.