The Jack Arch doesn't have a traditional 'arch' shape at all. It's flat. While the Jack Arch requires more mass surrounding the opening to stop any spreading, it is still amazing to see tons of stone suspended in air this way. I took this photo of this stone house with the Jack Arch windows still in tact after all the wooden frames were gone. Sometimes we imagine it's the wood or steel lintels that keeps the stonework up. Not so.
I have never built a dry stone Jack Arch. I wonder if anyone has?