I've noticed that email client, Thunderbird, uses an arrow pointing up as an indicator that the column is being sorted in descending (newest to oldest) order.

Most applications use an arrow pointing down to indicate sorting in descending order. This is peculiar to me from a usability perspective. Which approach is more intuitive to the user?

Here is a screenshot of Thunderbird sorting in ascending order (oldest to newest):

On the other hand, phpMyAdmin (for example), uses the opposite, more common approach - using an up arrow for ascending (and down arrow for descending):

You'll see this approach in just about every application you use. phpMyAdmin's approach feels more intuitive to me, but that could just be because I am used to seeing it displayed this way.

The nature of an arrow

Let's consider what an arrow is really indicating.

Looking at the shape of an arrow, we can see that it is simply a triangle, which has a wide base, and narrows in to a single point. It feels almost like a funnel - we start with a lot, and end up with very little.

Consider "greater than" and "less than" symbols:

> <

These tend to indicate the same comparisons - whatever is on the left is greater than or less than whatever is on the right.

In the case of sorting, though, arrows are not used to indicate quantity or amount, but rather direction.

Directional subterfuge

There are many types of directions, when it comes to applications and data:

• Numerical
• Alphabetical
• Chronological

The direction depends on the nature of the data. If it's a number, the direction refers to numerical order. If it's words, the direction refers to alphabetical order. Dates - direction refers to chronological order.

Column sorting arrows, however, point in a spatial direction. That is, up or down; left or right. We often perceive "up" as indicating the future, or more of something. "Down" usually indicates the past, or less of something. "Left" and "right" can also indicate past and present; less or more.

In the case of our application sorting arrow, the indication is that we are pointing towards the start or end of the data stream.

Everything has a start and end, which is often indefinite in either direction. If you picture the data stream as a single line, the arrows indicate which direction on that line you are looking.

Those that associate a down arrow as pointing to the past will more likely be comfortable with a down arrow for descending sort order (as most applications display it).

Here's how I perceive arrow indicators, and their directional meaning:

I would think this perception is natural to most people. Thunderbird, however, thinks the opposite.