Many comma conventions are observed for the sake of clarity.
Commas are used in dates to set off the day of the week or the year from the rest of the date. A second comma is used after the year to separate it from the rest of the sentence. If the year comes at the end of the sentence, the second comma is not needed.
On the wintery morning of January 27, 1933, my father was born.
He was born January 27, 1933.
His last birthday was Wednesday, January 27, 2016.
If you use only the month and year, it's not necessary to use any commas.
January 1933 was a busy month for my grandmother.
Commas are not needed for dates written in the European format, in which the day comes first and the month second.
My father was born on 27 January 1933.
When writing a city and state, the state is set off from the rest of the sentence by a pair of commas. If the state comes at the end of the sentence, the second comma is not needed.
I was visiting Naples, Florida, last weekend.
Last weekend I was visiting Naples, Florida.
When writing an address all on one line or in a sentence, use a comma before the following elements: the apartment or suite number, the city, and the state. It's not necessary to use a comma before the zip code.
Her address is 3425 Stone Street, Apt. 2A, Jacksonville, FL 39404.
If using a person's name along with the address, place a comma after the name.
Jane Doe, 3425 Stone Street, Apt. 2A, Jacksonville, FL 39404
If the address is written on three lines, as on an envelope or at the top of a letter, no commas are needed between the lines. A comma is still needed before the apartment or suite number and before the state.
3425 Stone Street, Apt. 2A
Jacksonville, FL 39404
Sometimes the apartment or suite number is written on another line. If that is the case, the comma before the apartment number is not necessary.
3425 Stone Street
Jacksonville, FL 39404
If a person's academic or professional title follows the name, it is separated from the rest of the sentence by a pair of commas. If the title comes at the end of the sentence, the second comma is not needed.
Jerry Mayor, MD, graduated from medical school twenty years ago.
Do you know Jerry Mayor, MD?
Do not use commas with titles that are part of a person's name.
Jerry Mayor Jr. is the son of Jerry Mayor Sr.
My son's name is Larry Miller II.
In numbers more than four digits long, commas separate the digits into groups of three. In scientific writing, the comma is not used in four digit numbers (thousands).
I have $4,500 in my bank account.
In larger numbers, the comma is always necessary, regardless of whether the number appears in scientific writing or not.
- In order to figure out where to place commas in a large number, count backwards in groups of three digits, not forwards.
Numbers used as street addresses, zip codes, telephone numbers, or years do not require commas.
29112 Cherry Tree Lane
Our zip code is 92775.
My father was born in 1965.
A comma is used to separate the greeting of an informal letter from the body. (A formal business letter requires a colon instead.)
Dear Aunt Sue,
How have you been? ...
A comma is used to separate the closing of a letter from the signature.
... I hope to see you again soon.