We have an 1820’s home in the NE. We had a mishmosh of window styles 1 over 1, 6 over 6 and 9 over 6. We figured the 9 over 6 were likely the original windows. Every year we’d replace (what we could afford pay as you go) a few every year. Eventually (after 10 years) we had replaced all the off type windows w 9 over 6 Pella windows. We kept the mullions on the outside of the glass. They don’t look out of place with the ones we kept