Part-of-speech tagging is the process of converting a sentence, in the form of a list of words, into a list of tuples, where each tuple is of the form (word, tag). The tag is a part-of-speech tag, and signifies whether the word is a noun, adjective, verb, and so on.

Part-of-speech tagging is a necessary step before chunking, which is covered in Chapter 5, Extracting Chunks. Without the part-of-speech tags, a chunker cannot know how to extract phrases from a sentence. But with part-of-speech tags, you can tell a chunker how to identify phrases based on tag patterns.

You can also use part-of-speech tags for grammar analysis and word sense disambiguation. For example, the word duck could refer to a bird, or it could be a verb indicating a downward motion. Computers cannot know the difference without additional information, such as part-of-speech tags. For more on word sense disambiguation, refer to the URL https://en.wikipedia.org/wiki/Word_sense_disambiguation.

Most of the taggers...