Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. A commonly known subtraction method is the borrowing method. To perform a - b using this method, b, the subtrahend, is written below a, the minuend, such that the digits of the two numbers are aligned in columns. When a digit of the minuend is smaller than the corresponding digit of the subtrahend below it, the procedure calls for borrowing one power of 10 from the digit of the minuend that is immediately to the left of the current digit. Then the value of the "lending" digit is reduced by 1. (More formally this is the decomposition algorithm because the "borrowed" power of 10 does not have to be and is never returned.) Anyone who subtracts using the borrowing method must do so from right to left, starting at the lowest-value digit and proceeding toward the highest-value digit. It is also necessary to remember to reduce the value of each lending digit by1 before subtracting the corresponding digit of the subtrahend from it. Because of these requirements, learning subtraction with borrowing is difficult for some students.