KDE User Interface Guidelines
This document is intended to provide a place where KDE application designers/developers can review user interface design principles. It is intended to complement, not compete with, the interface standards. Design principles are not the same as standards; I view standards as something that can be measured and enforced. I like to think that the standards would be motivated by the principles. The standards should be defined so that a given application can be reviewed against them to determine its degree of compliance. However, our goal here is not to provide measurable standards, but rather to encourage better design and improve usability. (2001-07-22)
The link address is: http://developer.kde.org/documentation/design/ui/index.html