A transposition error occurs when an amount is recorded incorrectly as the result of switching the positions of two (or more) digits. The switching of the positions causes a difference (between the recorded amount and the correct amount) that will be evenly divisible by 9. For example, if 986 was the correct amount, but it was recorded incorrectly as 968, the difference between the two amounts is 18. Since this difference is evenly divisible by 9 (18 divided by 9 = exactly 2), the mistake is described as a transposition error. Another example of the many possible transposition errors is recording 3662 as 3626. In this situation, the difference between the correct and incorrect amounts is 36. Because the difference is evenly divisible by 9 (36 divided by 9 = exactly 4), this indicates that the error could be the result of transposing digits.
If you are faced with finding a difference between two amounts, you might save some time by first dividing the difference by 9. (A few differences that are evenly divisible by 9 include: 9, 18, 27, 36, 45, 54, 63, 72, 81, etc.) Knowing that the error could be a transposition error may lead you to first review the more relevant amounts.