Should you use 'single quote marks' or "double quote marks"?
In other words, both are correct. Follow your own company style guide, and above all, use the same type of quote marks consistently.
So what happens when you need to write a quote within a quote?
This is the only time you will ever use both types in a document.
A. If single quote marks are your style, enclose the entire quote in single quote marks, and put the shorter quote in double quote marks:
'Keats wrote "Ode to Autumn" in 1819,' wrote Maria.
B. But if double quote marks are your standard style, do the reverse:
"Keats wrote 'Ode to Autumn' in 1819," wrote Maria.
In no other situation should you switch between single and double quotes.
Contented online course in Painless Grammar has dozens of similar tips to make your writing easy.