Here is a great video from Sal Dibiasi explaining Tile Leveling Systems.
Tile Leveling systems explained, when and why they should be used to reduce or even eliminate lippage between tiles. Lippage is when one edge of a tile is higher than the adjacent tile. Porcelain tiles are getting larger and larger as time goes on, any tile with one side longer than 15" (38cm) is considered a large format tile. These large format tiles are usually not flat, the manufacturing process cause the tiles to bow as they are made, some are bowed more than others, but all will have some degree curve to them, and the longer the tile, the more it becomes a problem. In this video I explain how this impacts the installation of the tile and why Leveling systems have become so numerous and adopted to help with the installation of these large tiles and how they reduce lippage of the installed tile. There are many types of systems, but all work in the same way to bring the tiles into plane by clamping the edges of the tiles together, whether the system is a wedge type, spindle or cap type, the principle is the same. The system I use the most is a wedge type system and is the T-Lock system by Perfect Level Master. All tile installed by Sal DiBlasi, Elite-tile Company, in the Boston North Shore area. This video contains affiliate links, which means I will receive a small commission if you click on the product link.