You can use single quotation marks (') and double quotation marks (") interchangeably to define a string constant. You must balance them properly, using the same type of quotation mark at the beginning and the end of the string. Use double quotes if your string contains a single quote, and vice versa, as in the following example:

DISPLAY "Always using the same type of quotes!".
DISPLAY 'Like this.'.
DISPLAY "But never like this!'.