This synthesis of the research on direct manipulation interfaces explores the nature of directness in computer interfaces and demonstrates that the concept of directness is complex, including two gulfs--for execution and evaluation--and two kinds of mappings--semantic mappings and referential distance. Examination of the complexities of the differences among interface styles shows the importance of visibility and sound in the performance of tasks, and a detailed analysis of the general attributes of cognitive artifacts is presented, including a new theoretical construct, the object-symbol. These analyses allow for a deeper understanding of the differences among existing human-machine interfaces and provide the background for the development of a new class of interfaces that promise superior performance. Twenty-four references are listed, and a draft checklist for the design of an artifact is appended. (Author/MES)