carriage returnsOnly when you want a new paragraph
On manual typewriters, the carriage was the cylinder that held the paper and scooted leftward as you typed. At the end of each line, you’d push a lever to move the carriage to the beginning of the next line. On electric typewriters, this lever became the carriage return key, which you’d use at the end of each line.
The terminology has stayed with us, but on a word processor, you only use a carriage return to start a new paragraph. (In HTML, you use a formatting tag to denote a paragraph—a carriage return has no visible effect.)
<p> … </p>
As with the word space, use only one carriage return at a time. It’s common to see multiple carriage returns used to add vertical space between paragraphs. Bad idea. If you want vertical space after a paragraph, use space between paragraphs.
“But it’s so much easier to type two carriage returns.” I know. But in long, structured documents, extra carriage returns create unpredictable consequences as the document is edited. Whatever time you save with the shortcut will cost you later.
What if you get a document that’s already littered with double carriage returns? Search-and-replace works with white-space characters too.
How to replace double carriage returns
Word Find and Replace (control + f) → Replace → More. Use the Special menu to put two Paragraph Marks in the Find what box, and one Paragraph Mark in the Replace with box. (Careful: you don’t want the Paragraph Character, which denotes the literal ¶ symbol.) Click Replace All.
OS X Word Edit → Find → Advanced Find and Replace → Replace → click triangle-shaped box in lower left to reveal the Special menu, and then continue as described above.
Pages This is awful, but it seems to be the only way. Display the white-space characters with View → Show Invisibles (or ⌘ + shift + i). In the document, select two contiguous blue ¶ symbols and do Edit → Find → Use Selection for Find. Then select only one of the blue ¶ symbols, and do Edit → Find → Use Selection for Replace. Then go to the Find panel (either Edit → Find → Find... or ⌘ + f). You won’t see the carriage returns you selected—don’t panic. Just click Replace All.