Comma separator for numbers with 4 or 5 digits?

Having gone to school back in the mid 80's to the early 90's, I pretty much sucked at math and English. However, one of the things I remembered was that the first comma in a large number was not inserted unless the number had at least 5 digits.

Example -

100

1000

10,000

100,000

1,000,000

However, I never really tried looking that up until now, when it came up in discussion. I can't seem to find a reference to this 'rule' or guidance anymore, and am wondering if - in my old age - I'm just completely making that up now.

Solution 1:

This is a question of style, and, if you are writing in a structured environment (e.g., school or work), you should follow whatever standards and guidelines are in effect in your situation.  But, as one somewhat official example, the U.S. Government Printing Office (GPO) Style Manual (a 467-page PDF) says, in Chapter 12. Numerals, section 12.14 Punctuation (page 289) the following:

The comma is used in a number containing four or more digits, except in serial numbers, common and decimal fractions, astronomical and military time, and kilocycles and meters of not more than four figures pertaining to radio.

although in preceding paragraphs they have indicated that commas should not be used in years (e.g., 2017).

Section 12.7 indicates that street addresses (e.g., “1727 St. Clair Avenue”) and telephone numbers are considered to be serial numbers (and thus should not have commas — presumably, even if they have more than four digits (and, yes, I know of a place where they have five-digit street numbers)).  It goes without saying that this also applies to postal ZIP codes.

Oddly, they say that “a 1,100-percent increase” and “an 1100-percent increase” are both acceptable.  They don’t explain; I guess the rationale is that “1,100” would be pronounced “one thousand one hundred” and “1100” would be pronounced “eleven hundred”.

And (again, without any explanation) they offer “Dow Jones average of 10500.76” as an example of correct usage.

Solution 2:

I've seen this "rule" before. In my opinion, it is a stupid rule.

"2017 graduates": is 2017 a year or a head count? Without that "rule", you could write:
"2017 graduates" (2017 is a year) or
"2,017 graduates" (2,017 is a head count)

Also, in tables, the numbers will not line up right if you include commas in some numbers ≥ 1000 but not others.

  9998
  9999
10,000
10,001

versus

 9,998
 9,999
10,000
10,001