There were several different meantone tuning schemes. Each involved slight adjustments to the sizes of the major thirds and fifths. As we saw above, pure major thirds, pure fifths, and pure octaves are not really compatible. So, with meantone, we end up with relatively few usable keys. About eight of the 12 major keys are usable.
Tempering is involved here.
Tempering involves adjusting some of the intervals. In this case, we adjust them, so they fit into an octave. That is what we did with major thirds, in Pythagorean tuning. We made them bigger, so that our fifths could be pure. This was a little more complicated, in a way.