When a number X is rounded to a certain number of digits to obtain an approximation x, the error is called the round-off error. For some authors this is X − x, and for others it is x − X. When a number is rounded to k decimal places, the round-off error lies between ± 5 × 10^−(k+1). For some authors, the error is |X − x| (see absolute value) and so, for them, it is always greater than or equal to zero.