Simplified Explanation of How Mail is Sorted

The Sorting Machine

The sorting system uses a machine that can place the mail in 1 of 220 buckets.

If you think about the problem, i.e., sorting the mail, not just to the appropriate mail carrier, but into the correct order of delivery addresses, this seems like quite a job. However, there is a trick. In the following example, there are only 9 buckets. But using the same process, the post office sorts the mail into 220 buckets.

How can you get an automatic sorter with only 3 bins to correctly sort the mail for 9 destinations?

You don't have 9 bins, you only have 3 bins, so doesn't the problem seem impossible? It's not.

It's solved by running the mail through the same machine twice.

The sorter can organize the mail in the correct order for over 48,000 final destinations. How does it work?

Let's use the example of a 3 bin system. In this case, the results are equally astonishing. By sorting the mail twice, the system is able to correctly organize the mail for up to 9 different locations. How is this, you may ask... here, we'll show you.

Lets say we have a bag of mail with addresses we'll represent as single digit numbers. We put the letters in a stack and find they are in this order:

2 3 6 7 4 5 9 8 1 2 4 3.  Some addresses get more than one letter.

First Sort

In the first sort, the system organizes the bins like this:

• Bin 1 gets:  1, 4 and 7
• Bin 2 gets:  2, 5, and 8
• Bin 3 gets:  3, 6 and 9

Processing the stack of mail as such give us the three bins with these pieces of mail in this order:

• Bin 1:  7, 4, 1, 4.
• Bin 2:  2, 5, 8, 2.
• Bin 3:  3, 6, 9, 3

The we stack the mail that has been sorted. Now the mail is in this order:

7 4 1 4 2 5 8 2 3 6 9 3

Second Sort

During the second pass, the mail is binned like this:

• Bin 1 gets:  1, 2 and 3
• Bin 2 gets:  4, 5, and 6
• Bin 3 gets:  7, 8 and 9

Processing the stack of mail as such give us the three bins with these pieces of mail in this order:

• Bin 1:  1, 2, 2, 3, 3
• Bin 2:  6: 4, 4, 5, 6.
• Bin 3:  7, 8, 9

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