Customary is an adjective that means commonly practiced or found, usual, based on tradition rather than law.
Customarily is an adverb that means usually or commonly or traditionally.
Adjectives modify nouns, whereas adverbs modify verbs or adjectives.
The customary coffee and donuts were served before the training class started.
Here the adjective “customary” modifies the compound noun “coffee and donuts.”
Coffee and donuts are customarily served before training classes begin.
Here the adverb “customarily” modifies the present passive verb “are served.”
So, if you can substitute “usual,” use the adjective “customary,” and if you can substitute “usually,” use the adverb “customarily.” The –ly suffix is the dead giveaway about most adverbs, which “adhere to the verb.”
Customary Quote of the Day:
“History warns us that it is the customary fate of new truths to begin as heresies and to end as superstitions.”
– Thomas Huxley, English biologist and agnostic, 1822-1895