Hard suitcases are also referred to as hard shell suitcases or hard sided suitcases. They are an alternative to the more common soft-sided suitcases. Before 5 years ago hard cases were commonly made from ABS(acrylonitrile butadiene styrene) or Polypropolene. These materials are strong and rigid and protected the contents of the case better than that of their soft counterparts, they also had fixed luggage locks making them more secure. For a long time these cases were popular until the airlines became stricter on luggage weight, the ABS cases in particular were very heavy, sometimes weighing as much as 10kg. Soon after the airlines tightened up their baggage weight limits the ABS cases became unpopular and are now rarely seen. The polypropolene cases were lighter and have still remained to today all be it in much smaller amounts. During this period hard shell suitcases had become unpopular and soft sided cases were the much more in demand cases. This has changed recently when luggage manufacturers began making suitcases from polycarbonate, this material has a great advantage over previous materials - it is lightweight. Since its introduction hard cases have again become popular.