The IEEE floating-point standard, supported by almost all modern floating-point units, specifies that every floating point arithmetic operation, including division by zero, has a well-defined result. The standard supports signed zero, as well as infinity and NaN ("not a number"). There are two zeroes: +0 ("positive zero") and −0 ("negative zero") and this removes any ambiguity when dividing. In IEEE 754 arithmetic, "a" ÷ +0 is positive infinity when "a" is positive, negative infinity when "a" is negative, and NaN when "a" = ±0. The infinity signs change when dividing by −0 instead.