And then there's the famously Helvetica-fetishising UI, whose typographical choice is seemingly more designed to exude mid-20th-century modernist chic and pander to the owner's self-image as a stylishly cool individual, rather than aiming for anything as gauchely utilitarian as legibility. While Helvetica is good for print and signage, or, indeed, larger sizes on the screen, there are more legible typefaces for use on computer screens (the Lucida family, shipped with Apple's own OSX, is a case in point). The Helveticolatry, though, pales into insignificance next to the Notes application's cutesy felt-marker typeface, which, whilst less cringeworthy than Comic Sans, is still somewhat ridiculous; all of a sudden, High Modernist chic gives way to kitsch.
Bonus link: Stephen Coles with a list of alternatives to Helvetica. (Note the complete absence of Arial in this list; it's a list of actual typefaces of typographical merit.)
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