One particular challenge with dollars is due to the fact each of the denominations are printed on precisely the same dimension notes, counterfeiters bleach small value notes and reprint them as high price bills. So they take a $1 bill, bleach it, then reprint it like a $a hundred bill. https://judahckmhd.eedblog.com/34007228/a-secret-weapon-for-where-can-i-copyright-dollar-bills