Wine can be stored in the unopened cases in which it is bought, but this is practical only for wines years away from being ready to drink, and very steady and sturdy shelves are needed if more than one layer of cases is to be stacked. A system which incorporates cases without lids on individual runners is particularly space-efficient. Wine racks take up more room but allow bottles to be kept horizontal and corks damp (not strictly necessary for bottles with screwcaps or sparkling wine) and retrieved individually. Racks with slots for individual bottles offer the best access for very mixed cellars (metal and wood is the usual combination of materials used for these racks), but larger compartments, or bins, can be used for larger quantities of bottles of the same wine.