I am putting up 15.5" shelves. I am planning on using 15"x12" shelf brackets. Which part goes under the shelf and which on the wall. For 45 years I have done half one way and half the other, so at least I am half right. Thought it might be time to ask. Anyone?

The shelves are replacing a recycled kitchen cabinet that has been on the wall for 16 years. Today the particleboard on the back disintegrated and the cabinet fell. Go figure.