What is the difference between standard and nonstandard amino acids?

Written and fact-checked by The Editors of Encyclopaedia Britannica

Amino acids typically are classified as standard or nonstandard, based on the polarity, or distribution of electric charge, of the R group (side chain).
The 20 (or 21) amino acids that function as building blocks of proteins are classified as standard.
Nonstandard amino acids basically are standard amino acids that have been chemically modified after they have been incorporated into a protein (posttranslational modification); they can also include amino acids that occur in living organisms but are not found in proteins. Among the latter is γ-carboxyglutamic acid, a calcium-binding amino acid residue found in the blood-clotting protein prothrombin.
The most important posttranslational modification of amino acids in eukaryotic organisms (including humans) is phosphorylation, in which a phosphate molecule is added to the hydroxyl portion of the R groups of serine, threonine, and tyrosine. Phosphorylation serves a critical role in the regulation of protein function and cell signaling.