An opon Ifa has a top, and the top is a face. Cut into the raised border of the tray is a head, and the diviner orients the whole object so that head is furthest from him. It is Eshu, the orisha who carries messages between the human world and the orisha, and he is at the top because he is addressed first. Nothing gets through if he is not. The part of the border where the face sits is called the oju opon, the face or the eye of the tray, and the tapper the diviner uses is struck toward it to open the work.
The rest of the border is carved too, usually as a band of figures and geometric fields, and it is not filler. What appears there is chosen, and a tray commissioned for a particular babalawo can carry his own concerns: his orisha, animals, human figures, sometimes a mounted rider. Trays run from the plainest to the most densely worked, and the finest of them entered European collections early, which is why Ifa was visible to outsiders as an art some time before it was intelligible to them as a system.
The working surface inside the border is deliberately the opposite. It is flat, it is plain, and it is meant to be wiped. The tray is not the oracle. It is the ground the oracle is written on, and it is cleared between readings the way a slate is.