The horizontal developing chamber, Fig. 6, can be used in either the normal or sandwich configuration for either conventional edge-to-edge or simultaneous edge-to-center development [25,26,28].The mobile phase is transported from the reservoirs (which require only 2 ml of mobile phase) to the sorbent layer by a glass slide;the liquid rises by surface tension and capillary forces. The mobile phase then traverses the sorbent layer by capillary action. When a plate is developed from opposite edges simultaneously, the chamber must be leveled to allow the two solvent fronts to migrate at the same velocity and meet precisely in the middle. A proper level setting also ensures rectangular solvent fronts are obtained. Turning the layer face down and removing the counter plate from the chamber allows operation as a normal chamber. The addition of solvent to the tray below the layer allows operation as a saturated chamber. This configuration also allows the layer to be preconditioned by adding appropriate solvents to the tray. Since capillary forces easily exceed those of gravity, the horizontal orientation of the plate has no significant effect on the mobile phase velocity.