Restricting The Vocabulary:
The properly formed vocabulary is like an inverted pyramid. All easy to learn communication system follow this pattern. The canonical vocabulary in the figure is comprised of the compounds, idioms and primitives. The Idioms combine compounds with the knowledge of the problem termed as domain knowledge. The Application specific commands and feedback forms the idioms. The input at this level would be commands like delete, create and draw.
The predictable output would be a scrolling effect, sorted output or dialogs. The next level is the compounds. They present the complex constructs created by combining one or more of the primitives. It is comprised of the generic input and output actions & symbols. The input at this level in GUI context would be actions like double clicking, clicking the button & manipulating the objects like push button and check box. The target controls would be edit fields and check box. Highlighting would be a possible output at this level.
Figure: The Canonical Vocabulary
The next level is the primitives. It is comprise of the minimum indivisible actions and feedback mechanisms. They form the atomic elements of which everything in the language is comprised. Input at this level would be commands like click, drag or key press. The expected output would be on the cursor and text. When the user interface design obeys the canonical vocabulary it will be an achievement.