Make Your Application Consistent
Make your application consistent with itself and with other applications, in both its appearance and its behavior. This is one of the most important design principles, and probably the most famous, but it is also frequently ignored. While this document serves as the basis for consistency between GNOME applications, you are encouraged to look at and follow other application's conventions where this document provides no guidelines.
Consistency enables users to apply their existing knowledge of their computing environment and other applications to understanding a new application. This not only allows users to become familiar with new applications more quickly, but also helps create a sense of comfort and trust in the overall environment. Most of the recommendations in the GNOME HI Guidelines are designed to help you create applications that are consistent with the GNOME environment and other GNOME applications.
A word of caution: a mis-applied or incomplete consistency is often worse than inconsistency. If your application includes an menu item for consistency, but it is always disabled because your application does not actually support Undo, this will reduce the user's trust in the availability of Undo in other applications on their desktop. Either make your application support Undo, or eliminate the menu item.