The level of protein structure in which regular hydrogen-bond interactions within contiguous stretches of polypeptide chain give rise to ALPHA-HELICES; BETA-STRANDS (which align to form BETA-SHEETS), or other types of coils. This is the first folding level of protein conformation.

Synonyms: Structures, Secondary Protein, Protein Structure, Secondary, Structure, Secondary Protein, Protein Structures, Secondary, Secondary Protein Structures, Secondary Protein Structure