usual relates to frequency, being customary (stresses the absence of strangeness ; applicable to whatever is expected happens in the ordinary course of events) common relates to being normal, not special (implies the lack of distinguishing or exceptional qualities ; it suggests usualness, everyday character or quality, or frequency of occurence; From: Merriam-Webster's Dictionary of Synonyms. html
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.