Have you ever used a faucet that had a hot water knob on the right side instead of the left?

Even if it’s well labeled, chances are you’ll turn the wrong handle a good percentage of the time. This is because controlling the faucet is something you usually do without thinking and your habit is to turn with your left hand when you want hot, not your right.

Consistency allows for habit, which saves time. Do a consistent navigation task a few times and after that you don’t have to think about it. It’s become habit, which means you can use more of your brain to think. The system backfires, however, when you unconsciously expect consistency, use habit, and are caught by surprise.

I often talk about the importance of consistent keyboard shortcuts across programs, because I use keyboard shortcut navigation more than mouse/toolbar navigation.

But consistency is just as important in toolbars.

The default order for many common groups of items is consistent across programs. For instance, Bold, Italic and Underline are commonly shown in that order. Left Justify, Center and Right Justify are commonly shown in that order. Style, Font and Size are commonly shown in that order. There’s a glaring problem, however, when it comes to the highlight and text color icons.

Microsoft Office toolbars put the highlight on the left and the text color icon on the right, while Google Docs and OpenOffice defaults put the highlight button on the right and the text color icon on the left.

The inconsistency makes it impossible to form a habit that’s useful across programs. If you get used to one way you’ll inevitably pick the wrong button when you’re in the program you’re not used to. If you regularly use a mix of inconsistent programs you’re likely to get things wrong fairly often.

In a world where people use multiple programs, inconsistent default order in groups of icons puts a larger-than-necessary cognitive load on folks. Worse, it makes habit a liability rather than an advantage.

It would be good for people if we had a standard order for related icons like Highlight and Text Color just as we have a standard order for faucet controls. The exact order matters much, much less than consistency across programs. Software is complicated enough already — we need to give people all the easy breaks we can.