Halliday & Matthiessen (1999: 4, 5):
We might refer to the Hjelmslevian notion of the "content plane" as incorporating both a grammar and a semantics (e.g., Hjelmslev, 1943). Grammar and semantics are the two strata or levels of content in the three-level systemic theory of language, and they are related in a natural, non-arbitrary way. The third level is the level of expression, either phonology or graphology. We can draw this model of language as in Figure 1-1.
Here the relationship is not one of 'consists of or 'is a subset of: theconcentriccotangential circles show the stratal environment of each level — thus lexicogrammar appears in the environment of semantics and provides the environment for phonology. This ordering of levels is known as stratification. We have used circles for all three levels to represent the fact that they are all based on the same fundamental principles of organisation: each level is a network of inter-related options, either in meaning, wording or sounding, which are realised as structures, based on the principle of rank.