PEMDAS is an acronym used primarily in the United States as a mechanism to pedagogically enforce the order rules of computational precedence. PEMDAS is explained as follows:
There are several obvious deficiencies with this model. For example, the fact that instances of the factorial operator are executed immediately after operations in parentheses and immediately before instances of exponentiation isn't indicated by this model. In addition, the ordering M-D (respectively, A-S) may appear to indicate that multiplication (respectively, addition) happens before division (respectively, subtraction), though multiplication (respectively, addition) receives the same precedence as division (respectively, subtraction).
It is not uncommon for American students to remember PEMDAS by way of any of a number of mnemonics, among the most common of which is, "Please Excuse My Dear Aunt Sally." Note, too, that there are analogues of PEMDAS in other countries as well, e.g., BODMAS (where B denotes brackets and O denotes orders) in the UK and BEDMAS in Canada (Pierce 2013).