It's been explained to me that the x-height of a typeface increases as a type gets heavier to recover some more vertical room and to prevent the type looking 'squat', also there it helps to balance the image optically when setting different weights next to each other. Correct me if I'm wrong.
I've observed that this doesn't happen also for the ascender and descenders, why is that?
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I get the impression this sort of treatment differs a lot from design to design, and from designer to designer; in the end it comes down to balancing it in a way that looks right. It’s a tradeoff: Making the weights look more like they are the same size in text will mean that they don’t line up when placed next to one another, which I guess can be undesirable in a face that’s more for display use.
In my understanding, the lowercase is usually most in need of enlarging because (1) it gets denser more quickly when the shape gets bolder, having a lot of horizontal elements and (2) it contains a lot of closed shapes, and the size of the counters is crucial in how large we perceive a shape to be. Most ascenders and descenders in Latin just being vertical sticks, making them bold within the available space isn’t as problematic, and their influence on the perceived size of a shape maybe also isn’t as big.
The above suggests that this problem is more pressing the lower the contrast is. As an aside, in my upcoming reversed-contrast text face I’ve had to significantly recompute the vertical proportions between weight masters. As the weight sits mostly on the horizontals, dark weights required a lot more vertical space – mostly in the lowercase; but leaving the ascender line in place would have shortened the relative length of the ascender so much that the ascender serifs didn’t fit in very well anymore. So I had to raise the ascender line a bit too. FWIW. There are interesting parallels to Arabic there, I was reminded of a talk I saw by Kristyan Sarkis where IIRC he mentioned having to adjust vertical metrics between weights rather significantly for Arabic because since the weight sits on the horizontals, the shape also grows in that direction when made bold.
On the other hand, it's not terribly useful or visually appealing for a heavy sans' square ascender to exceed the cap ht, for text or display use, while it is often invaluable, optically, to have a thin sans' square ascender, in the same family, exceed the cap ht, sometimes the cap overshoot, and sometimes, if it is a small enough master, thin enough weight, and wide enough body, the ascender seems to want to go on up forever.
I think this is because we are trying to make upper and lowercase heights that appear consistent across weights. And that means extenders and under/overshoot must compensate for size, weight and width, at least. And... what Nina said about reverse stress in Latin, and normal stress in Arabic, becomes super-stressful to vertical metrics in many CJK designs just beyond bold.
http://web.archive.org/web/20100103041624/http://typophile.com/node/65057
If the Bold has a higher x-height than the Regular, then should the alignment zones be standardized throughout the family, based on the Bold?
That’s what I did here (Regular outline pasted into Bold font).
I also recall that in making the Bold I increased the weight—e.g. Make Parallel Path—of the Regular by an amount that conveniently increased its x-height. (That was a separate process than increasing the thickness of the vertical stems.)
Isn't this what the Postscript Family Blues are for? My notes say those zones are used if the difference is < 1px.