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Like other similar pairs, ‘altogether’ and ‘all together’ have been used interchangeably by some speakers of English over the years, especially in writing (because we can still cover up our errors in speech sometimes). Whatever reasons we have for confusing these words, we can easily understand the difference, and this is it:

Altogether

This adverb simply means ‘in total’, ‘completely’, ‘entirely’ or ‘all in all’. Think of it as two words that have been combined to form a total and complete entity—just like its meaning.

Examples:

1. Each of the two houses has five rooms, so they have ten rooms altogether (in total).
2. The profit for the consecutive years was 10 million naira altogether (in total).
3. The game continued for another five years before it was abolished altogether(completely).
4. The performance was altogether awesome(entirely, completely, all in all).

All together

This phrase is used to mean ‘with each other’, ‘all in one place’ or ‘in a group’. To remember its meaning, think of it as two separate words following each other so that they can be in one place. This phrase is similar to ‘together’.

Examples:

1. She was happy to bring her friends all together.
2. We must put these ideas all together and present it as a proposal.
3. Put your clothes all together in the washing machine.
4. Let’s say it all together now: ‘We’re good to go!’

Altogether, these confused words cannot be used in place of each other because they are just not all together.

in simple words

All together means everyone or everything together

Altogether is an adverb and means "all in all," "all told," or "completely."