Embodiments of a key system for filters and their connecting heads/holders are shown. The filter cartridge and its holder each have a keyed surface, one being a protruding "key" and one being a recessed "lock". Cooperation of these keyed surfaces is required in order for the filter cartridge to be installed in the holder, so that mismatched cartridges cannot be installed into the holder, for example, to prevent a particular type of cartridge from being placed in a filtration or other process where it would be inappropriate, or undesired. The keyed surfaces are selectively locate-able preferably at different circumferential locations on a perimeter of the filter and a corresponding location on a perimeter of the head/holder. The perimeter may be, for example, on an outer shoulder surface of a filter and an inner surface of a valve head, or on outer and inner surfaces of connectors that provide a liquid seal between the filter and the head/holder.