> with TeX's standard settings, the inter-sentence space has the same natural width as the inter-word space
That's not quite accurate; the inter-sentence space does have a slightly larger natural width as well as more stretchability. It's right there in Chapter 12 of The TeXbook:
> "Plain TeX puts extra space at the end of a sentence; furthermore, it automatically increases the stretchability (and decreases the shrinkability) after punctuation marks."
Specifically, in the default cmr10 font, a normal inter-word space is 3.33333pt, and an end-of-sentence space has an additional 1.11111pt (before any stretch or shrink is applied).
Oh oops, sorry about that, thanks for the correction — was doing something custom recently, and got myself confused. So just for completeness: at the default settings, for cmr10 (a 10pt font),
• inter-word space has natural width 3.33333pt, stretchability 1.66666, shrinkability 1.11111.
• inter-sentence space has natural width 4.44444pt, stretchability 4.99997, shrinkability 0.37036.
So the net-effect is that, for example:
• for a line set at natural width: inter-word space is 3.33333pt and inter-sentence space is 4.44444pt (so it's 33% larger)
• for a loose line, stretched by ∛2 (badness 200, the maximum without complaining with the default setting of \tolerance): inter-word space is 5.43319pt, inter-sentence space is 10.7440pt (so it's 97% larger)
• for a tight line, shrunk by ∛2 (badness 200, the maximum without complaining with the default setting of \tolerance): inter-word space is 1.93342pt, inter-sentence space is 3.97782pt (so it's 106% larger)
So only for very loose or tight lines does the inter-sentence space get close to "two spaces".
That's not quite accurate; the inter-sentence space does have a slightly larger natural width as well as more stretchability. It's right there in Chapter 12 of The TeXbook:
> "Plain TeX puts extra space at the end of a sentence; furthermore, it automatically increases the stretchability (and decreases the shrinkability) after punctuation marks."
Specifically, in the default cmr10 font, a normal inter-word space is 3.33333pt, and an end-of-sentence space has an additional 1.11111pt (before any stretch or shrink is applied).