mugged

All the mugs I own are of different shapes and sizes and I've been musing about ways to make them more easy to carry, without using a tray, so I've been looking at designing one with a non slip rubberised area, that stops the other two from slipping and spilling, or designing mugs with ribs, that lock together when picked up, to make it easier to carry 3/4 cups to a table without a tray.