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all together, altogether

All together means as a group:
            All together they weighed 40 pounds.
            We found the books all together or
            W
e found all the books together.

Note: All together can only be used if it is possible by rephrasing to separate all and together.

Altogether means entirely:
            The trip was altogether worthwhile.
            Altogether the United States uses 339 billion gallons of water each day.

As a noun, one's birthday suit, presumably all in one piece, uses altogether:
            The Style Committee declined to pose in the altogether (meaning nude: note "the").