When two plates or beams come together into the same plane on a post, there is often not enough thickness in the post for each plate to connect to it with a tenon. If you put a tenon on the end of both beams (and a mortise on each side of the post), you risk taking too much “meat” out of the post that you create the mortise in.
A beautiful and strong solution to this issue is to use spline joinery in place of mortise and tenon. A through-mortise is cut into the post (which can be smaller than a typical mortise, and therefore not taking as much wood out of the post). Each beam or plate has a matching open mortise cut into it. The spline is a thin piece of a suitable hardwood that is passed through the beam’s through-mortise, and the open mortise on each supporting beam. The spline then gets pegged through the beams to secure everything together.
The size of the spline can vary, but for a typical 8x8 post it is usually made from a 2x6 and can be in the neighborhood of three feet long. It makes a strong connection that resists the tensile forces within the beams. The bonus is that the spline and its pegs make for a beautiful detail in the frame.