>> numerals don't spell a word as letters do.
> I'm not so sure. If you see a sequence of numerals
> often enough (like "911") then they become a bouma
> just like letters.
Under your theory, all words and lines of text produce boumas (boumae?
boumai?), but very few fixed and recurring numbers do. Some that come
to mind are:
911 - police emergency or attacks in 2001
800 - toll free telephone area code number
*if followed by the telephone number itself*
1776 - four-digit numbers without a comma are dates;
but it's becoming more common to see
four-digit numbers for values, not dates
000-0000 - General form for telephone numbers
000.0000 in the US
00/00/00 - General form for dates expressed as numbers
00/00/0000 - Alternate to above after Y2K
Consider the convention in blackletter German publishing of setting
'italic' text as letterspaced text. That breaks the flow, but it is
also the prevailing convention. For that matter, italics break the flow
so the reader can see the distinction the author want to convey. I
believe numbers and numerical values, being different from words,
should stand out slightly, at least when set with lining figs.
But I'm a fan of OS figs, and so I don't object as strenuously, maybe
because the 1 and l aren't so easily confused (although now the 1 looks
like an i--an i for an l, eh?).