In a message some time ago, Wolfgang suggested that \def\ {\thinspace} would serve to get non-expanding or non-stretchable spaces after periods that occur within sentences and in bibliographies. When I discovered that, while this worked well in limited contexts, in a lengthy bibliography which relied on the period as the main form of punctuation it lead quickly to numerous line-breaking problems. So Wolfgang has now has kindly proposed \def\ {\hskip.1666em\relax} since \hskip is a TeX primitve and mentioned in every (plain) TeX manual. And I gratefully confirm that this works very well. Alan