Groups of storage devices are used to store all kinds of values in a computer. Of course, every computer has a main memory, but there are other groupings of storage as well, even in the ALU. These groupings are often made of flip-flops and are called registers.

Registers usually store a binary number, hence the ordering of the flip-flops is important. We humans tend to think of binary numbers being stored in the same way we commonly write them on paper, namely left to right. Decimal numbers are stored in the same way. For example:

90356

represents ninety-thousand, three hundred fifty-six. This is a lot larger than 65309, which is what we get if we read from left to right.

The rightmost digit is called the least significant digit while the leftmost digit is the most significant digit. This terminology comes about because changing the value of the least significant digit changes the number the least, whereas changing the value of the most significant digit changes the value the most. For instance, changing the 6 to 7 in 90356 gets us a value that is only 1 greater than 90356, whereas changing 9 to 8 causes a decrease of 10,000.

Binary numbers are also written from left to right, so they have a least significant bit and a most significant bit. These are often abbreviated as LSB and MSB, respectively. The flip-flop that stores the LSB is called the least significant flip-flop. There is also a most significant flip-flop that stores the MSB.