How to mix fonts in a way that doesn't change line spacing?
There are two problems here. The first is that the 'point size' metric refers the total height from the top of an ascender to the bottom of a descender (and a little bit of space beyond that): in traditional metal type, 12pt was the size of the lead block that the letter was 'carved' out of. So when you put each row of letters down, they line up in multiples of 12pt. So 6 lines of 12pt text uses 1 inch of the page.
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Different typefaces have different x-heights -- the height of letters like a o e, so that some appear to be much larger than others on the page. Also, digital typefaces have complicated things, making the measurement slightly more arbitrary.
The second problem is Page's typography. The default line Spacing is set to use '1 Line'. If you change that to use Exactly 14pt (or whatever), then the problem will go away.
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