Stratification: Orders Of Abstraction

Halliday & Matthiessen (1999: 4):

Here the relationship is not one of ‘consists of’ or ‘is a subset of’: the [cotangential] 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. … 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. … 

Language, therefore, is a resource organised into three strata differentiated according to order of abstraction. These strata are related by means of realisation.