Because you can't depend on your spellchecker to know the difference between the homophonesall together and altogether, you need to be able to tell them apart. 

Definitions

The phrase all together (two words) refers to people or things gathered in one place or all acting together.

The adverbaltogether (one word) means entirely, wholly, or in all.

Examples

• "They came from all parts of Peru. They had never seen each other before but they were all together now, lined up in front of the cement hulks whose insides they had not yet seen."
  (Mario Vargas Llosa, The Time of the Hero, trans. by Lysander Kemp. Grove Press, 1966)

• "I saw I was losing the conversation, if in fact I had not already lost it altogether."
  (Maya Angelou, Mom & Me & Mom. Random House, 2013)
• When 50 Elvis impersonators appeared all together at Windsor Palace, the queen observed that this was "altogether inappropriate."

Usage Note

"All together is used only in sentences that can be rephrased so that all and together may be separated by other words: The books lay all together in a heap. All the books lay together in a heap."
(100 Words Almost Everyone Confuses and Misuses. Houghton Mifflin Harcourt, 2004)
 

The Lighter Side of Altogether and All Together

Ted Striker: I flew single engine fighters in the Air Force, but this plane has four engines. It's an entirely different kind of flying altogether.

Rumack and Randy [together]:  It's an entirely different kind of flying.
(Airplane! 1980)

Practice

(a) The clowns stood ______ at the front of the chapel.

(b) Despite their painted smiles, the clowns' sorrow was ______ clear.

Answers to Practice Exercises

Glossary of Usage: Index of Commonly Confused Words