shoulder girdle vs pectoral arch

shoulder girdle and pectoral arch both are nouns.

Word	Noun	Adjective	Verb	Adverb
shoulder girdle	Yes	No	No	No
pectoral arch	Yes	No	No	No

As nouns, shoulder girdle and pectoral arch are synonyms defined as:

shoulder girdle and pectoral arch: the bony arch formed by the collarbones and shoulder blades in humans

shoulder girdle (noun)	pectoral arch (noun)
the bony arch formed by the collarbones and shoulder blades in humans	the bony arch formed by the collarbones and shoulder blades in humans