The one number most people have memorized is their Social Security number. Yet you might be surprised to learn that there are some hidden secrets behind the nine digits that make up your unique identifying number. Let's take a look at the hidden code that lies behind many people's Social Security numbers.

Social Security numbers through history
Throughout nearly the entire history of the Social Security system, the Social Security Administration followed some general guidelines in deciding what any given person's Social Security number would be. The first three digits of the Social Security number corresponded to the location of the Social Security office that issued the number. Typically, that office was close to where you were born, and so people born in the same general area tended to have either the same first three digits in their Social Security numbers or have those first digits be extremely close to each other. By contrast, those born in areas that were far away from each other tend to have very different Social Security numbers.

That's not a hard and fast rule, however, because there's no absolute requirement that you file for a number in the place where you were born. Indeed, for early Social Security recipients, initial applications likely happened wherever they happened to live when they applied at some point during their adult lives. In some cases, that would have been far from their birthplace.

In general, the numbering of the first three digits of Social Security numbers roughly followed the same pattern that the U.S. Postal Service used for zip codes. You'd typically find lower numbers in the New England and the Northeastern U.S., with numbers steadily rising as you travel southward and westward. Yet because the SSA only used numbers up to around 586, those codes almost never matched up exactly to a given zip code.

The middle two numbers in the Social Security number made up the group number, which reflects the order in which the SSA assigned Social Security numbers to new applicants. Yet for whatever erason, the SSA didn't just start at 00 and go up the list. Instead, they started with odd numbers between 01 and 09, and then proceeded with even numbers from 10 to 98. Only then did they come back to fill in the even numbers from 02 to 08 and the odd numbers from 11 to 99.

Finally, the SSA assigned the last four numbers as a serial number, following a strict numerical order pattern. The lower the number, the earlier in that particular area and group combination the applicant was.

Source: Wikimedia Commons.

The move toward random Social Security numbers
Unfortunately, for those born recently, the secrets of the Social Security number have largely gone away. In 2007, the SSA gave public notice that it intended to abandon its previous method for choosing Social Security numbers and instead to go to a random process for assignment. The SSA followed through with that change in June 2011.

As a result, several things happened. First, the geographical significance of the first three digits of Social Security numbers disappeared. Second, randomizing allowed the SSA to use some previously untapped area numbers, such as 000 and the numbers in the 900s.

Finally, the so-called High Group List that the SSA published stopped getting updated. That might not sound like a big deal, but it forced banking institutions that had counted on that list to help ferret out fraudulent Social Security numbers to use different processes for identity theft. As a result, banks from the size of Bank of America (NYSE:BAC) on down to the smallest local institutions had to make changes to their verification procedures for customers.

With only the group of young children born since 2011 having randomized Social Security numbers, though, the hidden codes behind most Americans' numbers still hold true. Going forward, though, an increasing number of people won't be able to find any secrets within their Social Security numbers.