1. The conceptual level is the user's "mental model" of the interactive system.
Two mental models for image creation are paint programs that manipulate
pixels and drawing programs that operate on objects. Users of paint programs
think in terms of sequences of actions on pixels and groups of pixels,
while users of drawing programs apply operators to alter and group objects.
Decisions about mental models affect each of the lower levels.
2. The semantic level describes the meanings conveyed by the user's input
and by the computer's output display. For example, deleting an object in a
drawing program could be accomplished by undoing a recent action or by
invoking a delete-object action. Either action should eliminate a single object
and leave the rest untouched.
3. The syntactic level defines how the user actions that convey semantics are
assembled into complete sentences that instruct the computer to perform certain
tasks. For example, the delete-files action could be invoked by a multipleobject
selection, followed by a keystroke, followed by a confirmation.
4. The lexical level deals vith device dependencies and with the precise mechanisms
by which users specify the syntax (for example, a function key or a
mouse double-click within 200 milliseconds).
This approach is convenient for designers because its top-down nature is easy
to explain, matches the software architecture, and allows for useful modularity
during design. Designers are expected to move from conceptual to lexical and to
record carefully the mappings between levels. This model was very effective in
the early days of computing, when command-line input vas common and
implementers had to vrite low-level syntax and lexical-analysis programs.
Since modern graphical user interface standards and toolkits have greatly
reduced the need to design or implement syntactic and lexical levels, this model
is less relevant today.