Statistics Definitions > Upper Tail and Lower Tail

## What is a “Tail”?

The “tails” of a distribution are, just like the name suggests, the appendages on the side of a distribution. Although it can apply to a set of data, it makes more sense if that data is graphed, because the tails become easily visible. For example, the following image shows the tails of a normal distribution:

The tail on the left is (perhaps not surprisingly) called a

**left-tail**; The one on the right is a

**right-tail**. A distribution doesn’t have to have both: it can have only one tail on one side.

## Upper Tail and Lower Tail

Although it’s more common to refer to the tails as being on the “left” or “right”, **this can pose problems if you aren’t looking at a graph.** In other words, if you’re dealing with a set of data, it won’t be very clear what data should go on the left or right of a graph. The alternate name for right and left tails, which addresses this issue, is** upper tail** and **lower tail**.

## Lower Tail (Left Tail)

The lower tail contains the lower values in a distribution. If you graph any distribution on a Cartesian plane, the lowest set of number will always appear on the left, because the lowest values on a number line are to the left. So, “lower tail” means the same thing as “left tail”.

## Upper Tail (Right Tail)

Similarly, the upper tail contains the upper values in a distribution. If you graph any distribution on a Cartesian plane, the highest numbers will always appear on the right, because the highest values on a number line are to the right. So, “upper tail” means the same thing as “right tail”.

**See also**:

Left or Right tailed? How to tell in easy steps.

Heavy and light-tailed distributions.

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