Some people use “XX” when writing out the check amount to ensure that the value isn’t changed. For example, “00/100” means “no cents,” but it could theoretically be changed to “100/100,” which would mean “one dollar.” A fraudster could try to fill out a check and change “00/100” to something like “10,000/100” (which would mean “one hundred dollars”), but no responsible bank would accept this. As such, there’s no need to use the “XX” designation.