 This lesson shows how J manipulates numbers alone or in arrays. First, we add a single number to a list of three. The programmer specifies the single number, the function, and the list. The J interpreter does the rest. And when lists are of equal length, here corresponding items are multiplied to produce a new list. We can also subtract corresponding items of lists, or divide corresponding items of lists. Finally, if the number of items in the arrays is not compatible, an error results. Also notice J's way of adding comments, NB period. After NB period, the rest of the line is ignored. Adding comments easily is vital for powerfully terse languages. And J is both terse and powerful. Now that you know how J calculates, both with single numbers and arrays, it's time to practice those skills.