Stylistically words can be subdivided into formal, neutral and informal layers. The formal and informal layers contain a number of subgroups. The main aspect of the formal layer is its bookish characher, which makes the layer more or less stable. The aspect of the informal layer is its lively spoken character. It is this that makes it unstable. The aspect of the neutral layer is its universal character: it can be employed in all styles of language and in all spheres of human activity.
Both formal and informal words have their upper and lower ranges. The lower range of bookish words approaches the neutral layer and has a tendency to pass into that layer. The same may be said of the upper range of the informal layer: it can very easily pass into the neutral layer. The lines of demarcation between informal and neutral, on the one hand, and bookish and neutral, on the other hand, are blurred. Neutral words, which form the bulk of the English vocabulary, are used in both literary and colloquial language.