While most lawyers don’t grasp Concision’s importance, all understand Clarity’s, but not factors subtly enhancing Clarity. Font selection is one of the most subtle factors that affect Clarity, yet Clarity should dictate font selection, whose first rule is use the most legible font; but many fonts are designed for legibility, without reliably detectable legibility differences among them. Font selection’s second rule, which also serves Clarity, discriminates between equally legible fonts by their degree of novelty: use a font different from the jurisdiction’s default but similar to it.
Long briefs demand legible fonts, but legibility remains important regardless of who inflicted the judge’s eyestrain. Most of the commonly used briefing fonts are not among the most legible. The once-dominant Courier New mimics a typewriter rather than achieving the legibility possible with modern technology, and a space-saving design compromises Times New Roman. Of three commonly used legal fonts, New Century Schoolbook, the U.S. Supreme Court’s choice, alone performs among the most legible long-document fonts.
As the Seventh Circuit Court of Appeals points out in its web site, the best font is one designed for book reading. (See "Requirements and Suggestions for Typography in Briefs and Other Papers.") At places, the Seventh Circuit’s rules are dated, as in the prohibition on sans-serif fonts, more modern-looking typography lacking fancy stroke endings. Although books aren’t yet printed in sans-serif fonts, research shows them equally legible.
You don’t know the judge’s font preferences, but you can rely on the principle moderate novelty attracts greatest interest. A markedly different font distracts, while one identical to the norm doesn’t help keeping the judge’s attention. New Century Schoolbook, for example, is close enough to Times New Roman, but Avant-garde isn’t.