Relating to or serving as an accompaniment; supporting or complementary in nature.
From accompaniment + -al suffix. Accompaniment comes from accompany, with -ment creating a noun for the thing that accompanies (like a musical accompaniment).
Musicians use this word to describe instrumental parts that support the main melody—it's the difference between the guitar solo and the rhythm guitar, and in life we do the same thing: sometimes we're the solo, sometimes we're the accompaniment.
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