Many old living rooms and dining rooms have dark-stained windows, and indeed, many of the beautiful casings, stools, aprons and inner-stops are made from oak – mostly red oak, but sometimes the even more opulent ray-flecked white oak. (Less common, but also often seen, are casings made of stained birch, which can be mistaken for mahogany.)
However, the sashes – the up and down moving wooden frames that surround the glass are normally not made of oak. Here in Minnesota, they are made of Northern White Pine.
The sashes were also stained dark to match the oak (or birch, or some other hardwood,) and because the sashes are the same color as the more visible casings, many people mistakenly believe their sashes are also made of oak. They are not. They are made of pine.
Why two different species? Resin-rich pine naturally resists moisture so it is ideal for exterior applications. (Sills, blind stops, exterior casings and drip caps also need to resist moisture and are therefore also made of pine.)
Hardwoods like oak and birch have the qualities of beauty and durability and are therefore appropriate for interior applications.