We propose an approach to design rationale (DR) that emphasizes supporting the design process in such a way that a trace of the rationale is captured with little disruption of the normal process. We describe a rhetorical method for design dialogue called IBIS (meaning "issue-based information systems") and two implementations of this rhetorical method: a graphical hypertext tool for conducting IBIS discussions called gIBIS and a simple indented text notation. We describe a field trial in an industrial setting in which the "low-tech" indented text IBIS was used to capture more than 2,300 requirements and design decisions. We also explore the implications of this experience for the design of computer tools that, like gIBIS, seek to capture DR nonintrusively.