The American Bankers Association standard is for a currency strap to contain 100 bills of the same value, except for $1 bills, which can also be arranged into straps of 25, 50, 200 or 250. The Federal Reserve uses bundles consisting of 10 straps of the same denomination.
The ABA Standard uses color-coded bands for straps to increase efficiency in counting. Each color is used for a different denomination and, in the case of $1 bills, for a different total amount.
The Federal Reserve guidelines for packaging currency includes require the straps to have only one band around them, and it does not permit any paperclips, rubber bands or sub-straps. Every 10 straps of the same denomination are tied into a bundle with tight-fitting rubber bands or zip ties.