A currency strap, also known as currency band or bill strap is a simple paper device designed to hold a specific denomination and number of banknotes. It can also refer to the bundle itself.http://www.philadelphiafed.org/education/resources/history-of-currency-counting/#1960s/70s

In the United States, the American Bankers Association

American Bankers Association

The American Bankers Association is an industry trade group and professional association representing the United States' banking industry...

 or ABA has a standard for both value and color, as shown below. Note that all bills greater than $1.00 only come in straps of 100 count. The colors allow for quick accounting, even when the bills are stacked, such as in a vault

Bank vault

A bank vault is a secure space where money, valuables, records, and documents can be stored. It is intended to protect their contents from theft, unauthorized use, fire, natural disasters, and other threats, just like a safe...

.

History

Bundling money together with a simple elastic or paper device is as old as paper currency itself. However, measured and standardized straps are a relatively new idea. For example, until the mid-70s, The US Federal Reserve counted bills by hand. That is, they employed 55 currency counters whose job it was to count as well as, by touch and feel, authenticate bills.http://www.philadelphiafed.org/education/resources/history-of-currency-counting/#1960s/70s

However, as the amount of currency in circulation increased, they found that they needed a more efficient way to count currency. To help the Currency Counting staff keep up, the Bank began strap-sorting the $1 to $20 notes. Straps were visually inspected and weighed against a counterweight equal to the paper mass of 100 genuine U.S. notes. Though this method proved more time efficient, it was not as precise, since some fit currency was classified unfit and destroyed, while some bills that should have been destroyed were returned to circulation.

ABA Standard (United States)

	Strap Color	Bill Denomination	Bill Count	Total Value
	Black	$1.00	25	$25.00
	Orange	$1.00	50	$50.00
	Blue	$1.00	100	$100.00
	Green	$1.00	200	$200.00
	Pink	$1.00	250	$250.00
	Green	$2.00	100	$200.00
	Red	$5.00	100	$500.00
	Yellow	$10.00	100	$1000.00
	Violet	$20.00	100	$2000.00
	Brown	$50.00	100	$5000.00
	Mustard	$100.00	100	$10000.00