My Grammar and I (Or Should That Be 'Me'?): Old-School Ways to Sharpen Your English

14.   In place of a word that has been omitted:
The room was cold, the bed hard.
15.   Before the word too when it means also :
Two pairs of ballet shoes and two tutus, too.

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Parenthetical commas can also add emphasis to the word too if it appears in
the middle of a sentence:
He’d  never  really  thought  about  where  to  put  commas  in  a  sentence,  but
then, too, he’d never thought much about punctuation at all.
16.   To emphasize an adverb:
I wrote it down, quickly, which is why I went to the wrong address.
17.   After greetings and before closings in letters:
Dear Sir,
Yours faithfully,
This  used  to  be  a  hard-and-fast  rule,  but  is  beginning  to  fall  out  of  favour.
Now the hard-and-fast rule is be consistent : if you use a comma after Dear
Sir , use one after Yours faithfully , too.