"the alphabet," late 13c., abece. Sense "rudiments or fundamentals (of a subject)" is from late 14c. From 1944 (in a "Billboard" headline) as a shortening of American Broadcasting Company. Related: ABCs.
1. The angles are equal, for ABC is equilateral.
2. Do you know your ABC?
3. a roving reporter for ABC news
4. If you want to acquire profound knowledge, you must start from the ABC.
5. Making a simple dress out of cotton cloth is easy as ABC.