Some times printing errors occur in printing notes which are printed in Pakistan. Some times these errors out of sight and the note start using in the local market. But most of the times these errors are checked and a new note take the place of that note, which is of X serial. This note is called X note or replacement note. This note is considered more value able than its face value for collectors who collect bank notes as these notes are very rear. X, X1, X2 and X3 serials have been issued so far.