I've seen single quotes for chars vs. double quotes for Strings in Java, but in sh, it's "Strong Quoting with Single Quotes" vs. "Weak Quoting with Double Quotes":
"When you need to quote several character at once, you could use several backslashes. This is ugly but works. It is easier to use pairs of quotation marks to indicate the start and end of the characters to be quoted. Inside the single quotes, you can include almost all meta-characters"
So some of us associate single quotes with proper const strings, with no variable expansion, command substitution or other interpreter hanky-panky.
http://www.grymoire.com/Unix/Quote.html#uh-1
"When you need to quote several character at once, you could use several backslashes. This is ugly but works. It is easier to use pairs of quotation marks to indicate the start and end of the characters to be quoted. Inside the single quotes, you can include almost all meta-characters"
So some of us associate single quotes with proper const strings, with no variable expansion, command substitution or other interpreter hanky-panky.