The spatium in typography and the rule on spacing between initials
Spacing (the Russian shpatsiya, from the German Spatium) is a typographical term for a thin plate used to make gaps narrower than the body of a type. As for the rule concerning the gap between initials, I find it very hard to observe, simply because the standard interval in that place strikes me as unsightly. A thin space, now, is quite another matter, but that belongs to another century, of course