A system with levels incorporates mechanisms to map different matrices to the same physical interface, thus allowing much larger vocabulary sets. A great deal of effort has been devoted towards further improvement of the language interface. One of the earliest techniques used was optimization of vocabulary sets. Optimization is based on the nonuniform access time to specific units in the vocabulary set. For example, in a simple row column scanning array, the access time to an item is proportional to its distance from the upper left hand corner of the display.
Optimization is possible because the human language is structured and rule governed.
The keyboard optimization discussed above relied on the fact that in general, letters occur with varying frequencies. Because 'e' occurs more frequently than 'z', it should be more accessible to the user. Letter sequence frequency has also been exploited in other ways such as in linguistic prediction. Predictive systems rely not only on general information about language, but also on information obtained from the user. To continue making progress in the design of better language interfaces, it is necessary to utilize techniques that take greater advantage of the structure of human language and derive more information from the device user.