In the Library of Congress Classification System, the world of knowledge is organized into 21 broad categories represented by one or a pair of letters of the alphabet.
A succinct example is the organization of knowledge in the sciences is represented by the Letter P. All Language and literature classes include the letter P. All specializations are paired letters, beginning with the letter P.
The single letter P represents Philology and Linguistics. PA=Greek and Latin language and literature; PC=Romantic Languages; PR=English Literature; PS=American Literature. See the list to the right for a complete explanation of the letter P class.
P1 - P1091 Philology and Linguistics
P87-96 Communication. Mass media
P94.7 Interpersonal communication
P95-95.6 Oral communication. Speech
P98-98.5 Computational linguistics. Natural language processing
P99.5-99.6 Nonverbal communication
P101-410 Language. Linguistic theory. Comparative grammar
P118-118.7 Language acquisition
P121-149 Science of language
P201-299 Comparative grammar
P215-240 Phonetics and Phonology
P301-301.5 Style. Composition. Rhetoric
P302-302.87 Discourse analysis
P306-310 Translating and interpreting
P331-365.5 Comparative lexicography
P368 Standard language
P501-769 Indo-European (Indo-Germanic) philology
P901-1091 Extinct ancient and medieval languages
Click this link for a Complete list of the LC Classification