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to garnish; to accompany (as a card does a gift)
to add to as support; to prop up
to accompany (as an aid, guide, translator, etc.)
お祝いの品に手紙を副えて贈った。
I sent a congratulatory gift with an accompanying letter.
料理に季節の花を副えて彩りを添えた。
Seasonal flowers were placed alongside the dish to add color.
プレゼントに手書きのカードを副えた。
I included a handwritten card with the gift.