Bracket...

I close you safe within me,
But you just want to get free,
Remember once I say good bye,
You all will suffer the multiply.

I keep you in me for long,
So that I am silent in the song,
Sometimes I am the background score,
While I am used for chorus more.

I may be round, box or brace,
I may have a different face,
But one thing I always signify,
I am the symbol for 'unify'.