So what makes a tyrannosaurid a tyrannosaurid? Well, there are a lot of very technical things that make them tyrannosaurids, but most of them can be summed up in less technical ways.
First and easiest to see are that tyrannosaurids are large, meat eating dinosaurs with only two
fingers. They have large teeth. The front teeth are D-shaped if you cut them in half horizontally.
Another easy thing to spot is the bones (squamosal and the quadrato-jugal) that push in from the
back of the skull into the last opening in the skull (the infratemporal fenstra). The foot has three
bones (metatarsals) that are held together in a strong union, with the middle one (metatarsal III)
being pinched off from view at the front. Dr, Holtz has named this the arctometatarsalian
condition. It is also seen in the ornithomimids and the troodontids (coincidence? I think not).
There are may other distinguishing features having to do with the pelvis, the femur, the sinuses in the face, skull bones, but they are well documented elsewhere (see reference section) and really beyond the scope of this page.