Calligraphic styling.

Have a look at what Lirico does. The gaps fill in at text sizes.

Also, it could deviate forms from
each other (for improved readability).

hhp

@hrant: do you refer to Scala or do you speak generally? Because I have doubts about that 6/9 pair, since it's basically the same rotated form.

In the case of Scala, opening up the counters definitely adds to its intended use. Martin Majoor conceived the typeface at the end of the 1980s with low resolution output devices in mind (such as the 300 ppi desktop laser printer). Hence also the slab serifs. So, a kind of ink trapping, indeed — or rather “pixel trapping”.

But the open counter of P has a much older, palaeographic origin. (Deviate the “Roman” (= Old Italic / Western Greek) Π (Pi) from the Greek Ρ (Rho).) Cfr the Trajan capitals, most Garamont interpretations, Palatino, &c.

Exactly. But with lighter/less pixels.