Last night, I was having trouble sleeping, so I did what people with blogs do: babbled semi-coherently about my interests (and maybe inadvertently insulted a nice guy, a corollary of John Gabriel’s Greater Internet F***wad Theory). Much to my surprise, however, my babbling does not exist in a vacuum: see exhibit A, in which Dan gets paid to ride my coattails, and exhibit B, in which Tim (the blogger I said was wrong about user interface consistencies on the Mac) delivers a response. Well, this time I will be nicer, though I’ll still disagree somewhat.

In his response, Tim points out that if every folder icon were to look the exact same, there would be no differentiating between them. Looking different helps users tell icons apart, and some styles help group related families of icons. If the analogy follows, then, it would make sense to vary the visual style of application windows across the Mac OS, with consistent styles grouping related families.

I agree with this in principle, including the part about consistency within families. Go ahead and make the metal tint a little darker on the pro apps, or do something fun with the iLife suite. But Tim’s analogy doesn’t quite accurately describe how application windows differ in their looks and—perhaps more importantly—their behavior.

A unified UI doesn’t mean dozens of identical application windows, impossible to tell apart at a glance. You can have the same style of metal frame around the window for Mail and iTunes and still tell them apart with relative ease thanks to other points of differentiation: type and orientation of buttons, layout of panes within each window, dimensions of each window, and additional displays (such as the digital playback display in iTunes, absent from Mail).

The graphical inconsistencies I’m discussing don’t just usefully differentiate applications: they add non-functional visual clutter. As Dan points out, odd visual quirks sometimes fail to serve any useful purpose. Graphic design isn’t just about variation for the sake of variation, but purposeful and restrained variation calling attention to important features. Broadly speaking, this is why posters and flyers made by amateurs tend to use a variety of silly fonts, but posters made by professional graphic designers strategically use very few fonts. When every element in a design is competing for attention, no element is actually drawing attention to itself.

Also consider that the frame around a window isn’t just an issue of visual function, but of behavioral function: you can drag a window around by clicking on its metal surfaces, which means that inconsistently-designed windows that only offer a thin metal menu bar have less clickable surface. This can be irksome when you’re using a trackpad, as I am right now, because it’s easier to use a UI when you can afford to be less precise with your cursor movements. (See the answers to this interaction design quiz, such as the answer to question 5, if you’re not sure what I mean.)

I’m not saying that every single application window has to look exactly the same. As Tim notes, “Really it’s not about making everything consistent or making everything inconsistent. Either extreme would not be usable.” I agree: some applications, like chat programs, potentially have so many open windows that a full metal frame around each looks unnecessarily heavy and takes up a lot of screen space. Actually, Apple points out just that in its own developer guidelines for windows appearance—and then it broke its own guideline in iChat. And that’s the real problem here: not that one type of window is always better than another kind, but that Apple doesn’t follow its own rules. (That, and the fact that brushed metal is kind of an eyesore after awhile.) Some variations are more useful than others, and unfortunately, I think Apple has erred on the side of the useless kind lately.

So, Tim: thanks for the exchange, for being nice, and for not being sore about the wombat crack. And, Dan: sorry about the coattails comment. Blogging makes me a jerk even with guys I know. I’ll have to propose some revision to Dr. Gabriel’s theory.