Why is "of" in the phrase "all of the" unnecessary?

While it's not incorrect to say "all of the" before a noun, it's often considered colloquial or informal. It can sound clunky to native English speakers.

It's widely accepted to say "all the" instead. This is because "all" is functioning as a predeterminer, and it comes with a different set of rules than when it functions as a pronoun or determiner.

Example

• Correct: She is going to all of the parks.
• Correct and better: She is going to all the parks.

Example

• Correct: He does all of the chores.
• Correct and better: He does all the chores.
  
  Because it's not necessary to use of, we recommend getting rid of it completely. Extra words bog down your writing and make it harder to read. Sticky sentences are sentences full of the most common words in English. These affect both the pacing and clarity of your writing.

Why is of considered unnecessary but not incorrect in this situation? It has to do with the function of all.

Sometimes, all functions as a pronoun. In these cases, of is required. If a sentence can be restructured with a different pronoun and not lose its meaning, all is probably functioning as a pronoun.

Example

• Incorrect: All them are boys.
• Correct: All of them are boys.
• Correct and better: They're all boys.

Other times, all functions as a determiner. A determiner includes words like a, an, the, those, etc. When all is a determiner, of is not used. If you are talking about a concept in its entirely or something that can't easily be quantified, all is probably being used as a determiner.

Example

• Incorrect: All of people are inherently good.
• Correct: All people are inherently good.

Example

• Incorrect: All of sunlight contains harmful UV rays.
• Correct: All sunlight contains harmful UV rays.

Of becomes optional when all is functioning as a predeterminer. A predeterminer further specifies a noun.

Example

• Incorrect: Friends are at a party. (Whose friends?)
• Incorrect: All friends are at a party. (Not specific; this implies all friends of all people are at a party.)
• Incorrect: All of friends are at a party. (All is a determiner.)
• Correct: All of my friends are at a a party. (Of is unnecessary, but all specifies how many of my friends.)
• Correct and better: All my friends are at a party.

Eliminate unnecessary words by deleting of when all is a predeterminer.