When a website asks for a book's ISBN number, are they usually talking about the 10 digit or the 13 digit one?

There's an "ISBN-10" and an "ISBN-13" on most books.


Do most sites want one over the other?|||Give the 13-digit number if you have one, and the 10-digit one otherwise. The industry switched from 10-digit to 13-digit numbers in 2007, when it became apparent that we would soon run out of 10-digit numbers.