I replied to something in a ten-year-old thread, and that was a mistake, if only because it turns out to now have an uninformative title.
Anyways, in that old thread, the claim was made (by Gerald Giampa) that Monotype has a finer level of spacing than foundry type. Hrant Papazian expressed disagreement.
I had something to add to this at this late date.
Foundry type, of course, inherently has no mechanical limitation as to its widths. To speak, therefore, of its limits, we have to cite a specific instance of actual practise. Fortunately, there is an obvious choice.
From information in the ATF type specimen books, but more importantly, from Legros' and Grant's Typographic Printing-Surfaces, I have surmised that regular ATF types, as opposed to their Self-Spacing Type and similar products, was designed so that all widths were a multiple of 1/4 point. Or, if one prefers, of 1/48 of the pica em, to use correct printing terminology.
Monotype, as we know, uses an 18-unit system. Thus, if we're looking at 9 point type, the widths of the letters in a font are all multiples of 1/2 of a point. If, instead, we're looking at 12 point type, the widths of the letters are all multiples of 2/3 of a point.
So it's obvious: Monotype loses.
Not so fast. Because there is another question to be examined. Fine spacing is a good thing to have, but how much of it is actually usable is another matter.
The IBM Selectric Composer is worth looking at in this connection. It is very limited typographically. Not only did the regular and italic versions of a given typeface have the same spacing, as with Linotype, but all typefaces were spaced the same way.
It had a nine-unit system. However, this mechanical simplicity was partially achieved through condensing the M and W; comparing the widths of most characters to those in Monotype Times New Roman, it becomes clear that the em was divided into 11 units. And, indeed, 11 point Press Roman on the Composer used a unit size of 1/72 of an inch, and looked well-proportioned. (Another of the typographic limitations of the Composer was that it had only three unit sizes, and so some point sizes of some faces were slightly condensed or stretched to fit.)
While there are problems with documents made with the Composer - the Selectric mechanism couldn't align characters quite precisely enough for typographic purposes - the spacing was not visibly limited. On the other hand, proportional-spaced texts produced on a five-unit IBM Executive typewriter are distinguishable at sight from printing.
And there's another thing about Monotype that I hadn't mentioned.
A standard adjustment on the Monotype caster, made whenever one puts in a matrix-case for a new typeface, is to specify the set width. The set width is adjustable in increments of 1/4 point, and while it is usually close to the point size of the type, it does not have to equal it.
Thus, for a given face, 10 point type might have a set width of 10 points, 12 point type might have a set width of 11 3/4 points, and 6 point type might have a set width of 6 1/2 points.
And so, once you have designed your typeface, you have an ultimate division of horizontal space into units of 1/18 of 1/4 of a point, even though you can't use them in a single size of a typeface.
What they do provide for you, though, is the ability to make the optical adjustment of making the face wider in smaller point sizes by uniformly changing the proportions of every letter.
With ATF foundry type, on the other hand, one can't scale every character by the same amount when going from 11 point type to 9 point type, for example. Widths that are not multiples of 11/4 and 9/4 of a point, respectively, also have to be used.
So the fact that one has more than 18 widths in the em - but 44 of them for 11 point type, and 36 of them for 9 point type, rather than the same number - is not much of a help (see the Selectric Composer) but is a positive nuisance.
Maybe the small discrepancies are such, though, that they can be ignored and left as tiny errors in the spacing between the letters. Otherwise, even if one were not making the smaller sizes wider, the widths of all the letters would still need to be individually adjusted for each point size just to fit into the mechanical limitations of the foundry system.