What's a good mnemonic for shell double vs. single quotes?

Single quotes are simple quotes, with a single standard: every character is literal.
Double quotes have a double standard: some characters are literal, others are still interpreted unless there's a backslash before them.

Single quotes work alone: backslash inside single quotes is not special.
Double quotes pair up with backslash: backslash inside double quotes makes the next character non-special.

"Double, double toil and trouble;
Dollars burn, and backquotes bubble."

Single, in contrast, is harmless.

Double-quotes (") are weak quotes- you need two of them, so they come in pairs, and variables punch right through them despite their numbers.

Single-quotes (') are strong quotes- they come singly, and variables are no match for them.