I've often wondered this myself. Is it just a convention? Is it because in a way, all the orders of operation are just "addition" ? addition is addition, subtraction is adding negative numbers, multiplying is addition, dividing is multiplying. But then if this were true, it would only work depending on which number system you have right? And if this was the convention, how did a middle eastern guy make this convention so widely known? Could somebody enlighten me on this question?