 Throughout the videos, you may have noticed me reading code like this as x equals 3, 7, 2, 5, y equals x sub 0. What's that sub all about? Let's take a look back at algebra, where you might have seen something like this. It looks a lot like our lists, except that the index number is written as a subscript. When computers first started being used for scientific purposes, programmers needed to represent lists like this, but they couldn't do subscripts on a teletype or punched cards. And they couldn't write something like this and read it as x1, x2, x3, because those are the names of independent variables, not part of a list. Those early programmers came up with a solution. They put the index number in square brackets to represent the subscript. In fact, in a language called Fortran, one name for lists was subscripted arrays. And that's why we use brackets for indexes and why we read them as x sub 1, x sub 2, etc. That's the story of sub.