In order to make the corpora more useful for doing linguistic research, they are often subjected to a process known as annotation.
An example of annotating a corpus is part-of-speech tagging, or POS-tagging, in which information about each word's part of speech (verb, noun, adjective, etc.)
The difficulty of ensuring that the entire corpus is completely and consistently annotated means that these corpora are usually smaller, containing around one to three million words.
Other levels of linguistic structured analysis are possible, including annotations for morphology, semantics and pragmatics.
Corpora are the main knowledge base in corpus linguistics.