Hope it's ok I rework your picture so we can all see what you are talking about.
No problem, the actual window is one I saw on Ebay, not the old single glazed
garage one I fished out of a skip. That window I took a load of photos of with the dimensions marked on it. Sort of had the idea to use it as a starting point, template.
Had a problem with puter had to reformat the whole drive, I wiped the pictures, could kick myself for that
I do remember though, the main frame had corner bridle joints and I think a T bridle joint in the center dividing stile. ( if that is the correct name ). The main frame had a rebate set into it, where the sash fitted. ( Again I think these are the correct terms, I've looked into what the componant parts of the variouis styles are called, still not sure
The ebay window frame has a rebate too. From what I've seen, I think I'm right, sometimes the actual sash has a rebate and not the window frame.
In my upload of the window, I'm wondering about the joint, there is a rebate on the main frame. I seem to recollect the skip window had a rebate too but used a corner bridle joint, which was plain to see.
I did a search on haunched bridle joint and got a few results, didn't have time to look into it in depth, perhaps the joint in the Ebay window is a haunched corner bridle joint? Whatever the type of joint used, what ways could it be made? With regard to the equipment I have at the moment.
I got the seller to send me several pictures of the window, but the corner has a card cover and can't see the joint.