Altogether means completely. All together means everyone or everything in one group. The plan failed altogether. The cast stood all together. Diction as a set is in word choice.
Entirely versus in a cluster
If you can swap in entirely, use altogether. If you can swap in as a group, use all together. One word versus two is the whole test.
Altar versus alter is another look-alike, in altar versus alter. Already versus all ready is in already versus all ready. Grammar overall is in ACT grammar rules.
The bottom line
Completely: altogether. As a group: all together.
Start practicing
Start with a free diagnostic, then drill your weak spots with 15-question quizzes and track how you're doing across Reading, English, and Math. Compare plans whenever you're ready to go further.
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