Without knowing the actual designer's rationale, “why” is a difficult question to answer. Nonetheless, in the case of your second example, the design of the three is strongly based on the design of the two, rotated 180° clockwise.

I think your characterization of the two numeral styles (really just the "1" and "3") is sound. What I think clashes between the second set and the lc letters is that "3" might be too old-fashioned to fit with that "a". But we need more to go by. And when it comes to "appropriate": everything is, sometimes. :-) So the best answer to any question kicks in: it depends. ;-)

hhp

Here's some more to go by..

A little back story -

This project links back to a previous post of mine, on revivals. What you're looking at is a project that started a few months ago as a loose revival of Haas Unica, but has since morphed into another neo-grot. I'm trying to reconcile the purity of a neo-grotesque against the present context, in a non-gimicky way, something that Atlas Grotesk in particular does beautifully.

So I am, for all intents and purposes, the designer. And my view on this particular issue was that I've always admired the angular aesthetic of the more geometric numbers, so would love to find a way to incorporate them into the font, without sacrificing the neo-grotesque spirit.

But I'm beginning to think that using the geometric is too anachronistic. But at the same time, it's hard to judge, since I am looking specifically at them, and know them so well, so I'll always see the difference! I'm finding objectively judging your work in type-design is impossible (its impossible in everything too!) :D

What do you think?

At this point I'm still ironing out glyphs & metrics in a book weight (its not kerned or hinted). I'm still a small fry in the big pond, so I'm slowly feeling my way through the project..

Numbers and Letters have different historical background.
The Latin alphabet (a, b, c, z) originally used the roman numerals (I, II, III, V, etc...).
The numbers that we use today (0, 1, 2, 3...9) comes from the Hindu-Arabic cultures, so you have a little more freedom when choosing the style of constructions (skeleton) you want to use, as long as they match the general spirit/style/voice of your font.

Thanks Pablo, that actually makes so much sense, and I feel like I should've known that!

Cheers

Seb

And to me freedom always leads to responsibility - in this case that of fixing the numerals to better belong with the alphabet (although that applies mostly to stroke contrast distribution, so not as much here).

hhp

Is it ever appropriate to match the numbers within the 2nd image with neo-grotesque letters?

I wouldn’t do it.
The neo-grotesque genre is a very narrow, very understated revival of a 19th century genre. Understatement is the whole point, but the non-traditional forms of 1 and 3 refute that, by increasing angularity and complexity. A revival that departs significantly from the model is a lost soul, unless it moves sufficiently far to become something else.

If your neo-grotesque were to include a binocular /g, an /l with a tail, and a straight-legged /R, then by all means.

Thanks for your advice guys, really appreciate it.

Seb