Horizontal Machining Centers (HMCs) are CNC mills with a horizontally oriented spindle.  These machines are frequently used in more production settings, typically making use of tombstones for workpiece holding.  Most modern horizontal machining centers typically have full 4^th axis or rotary 4^th axis indexing for proper positioning of the workpiece.  The design of the horizontal machining center allows gravity to assist with chip removal.  Through spindle coolant to further assist with chip removal is also very common.  

As these machines are often used in production environments, pallet interchanges to allow setup while the machine is in operation are common, as are material handling systems and pallet pools.