Transitional elbows can help you make changes during a project if necessary.
Knowing how to make transitional elbows gives you the flexibility to make changes in direction and elevation in one fitting. You didn't see too many "old-time" sheet metal workers making two fittings when they could get by with one.
Why? Because that's what journeyman pattern-cutters did. They knew the art. And with the use of parallel or radial lines or triangulation, they could manipulate a sheet of steel into anything. So can you. This is all relatively simple stuff. Here's how it's done.