In the left picture is the mortise and tenon from a portion of the original floor.
In the right picture you can see the new wood members being prepared with mortise and tenon joints. The mortise is the rectangular hole cut into the member in preparation for a tenon, also called a tongue or tang, to be inserted from a corresponding member. The tenon will then be secured to the mortise with an oak peg (not seen in picture).
The lumber used for this project is Southern Yellow Pine.