A string is any sequence of characters such as letters, digits, spaces, punctuation symbols, and control characters such as tabs, carriage returns, and newlines. Most programming languages support the use of string constants, which consist of literal characters between quotes.

C uses double quotes to enclose a string:

"What is the radius of the circle?"
        

Fortran uses single quotes to enclose a string:

'What is the radius of the circle?'
        

In C, we can also enclose a single character between single quotes:

putchar('x');
        

A character between single quotes in C is not a string, however. It is a single character, whereas a string is an array of characters. They are different data types that are not interchangeable.