A 'star' sheet is used to replace the imperfect sheet. When an imperfect sheet is detected during the manufacturing process after the serial number has been overprinted, it must be replaced with a new sheet.
On some notes, a star appears in place of the last letter. The letter O is not used because of its similarity to the digit 0, and the letter Z is not used because it is reserved for test printings. At the time of a series change, the suffix letter returns to the letter A and repeats the cycle. The last letter advances through the alphabet when all eight character serial numbers have been printed for a specific Federal Reserve Bank within the same series. The first letter of such a serial number identifies the Federal Reserve Bank (FRB) which issued the note since there are 12 FRBs, this letter is always between A and L.