Keyed system for connection of filter cartridge to filter holder

A filter for liquid for connection to a filter head having first and second keyed recess structures that are diametrically-opposed 180 degrees from each other, the filter comprising:

• a generally cylindrical housing containing media for filtration or treatment of liquid, the housing having a top end, a bottom end, a central axis between said top end and said bottom end, and an axial sidewall, an upper corner connecting the top end and the axial sidewall, and a neck upending from the top end at or near the central axis and containing fluid connections for connection to a filter head;
• wherein the housing further comprises a first protrusion structure having a top extremity and a bottom extremity, and comprising a first set of three key protrusions protruding radially outward from a circumferential surface between the central axis and said upper corner, wherein the three key protrusions are separated by spaces each extending entirely through the first protrusion structure from the top extremity to the bottom extremity, the first set of key protrusions all being within a range of 30-70 degrees on the circumferential surface;
• and the housing further comprising a second set of key protrusions protruding radially outward from the circumferential surface, diametrically-opposed 180 degrees from said first set, and the second set of key protrusions being within a range of 30-70 degrees on said circumferential surface;
• wherein each of said spaces separating the three protrusions has a circumferential space length, and each of the three protrusions of said first set has a circumferential length that is three or more times said space length, whereby all three protrusions of said first set fit in the first recess structure of the filter head to connection the filter to the filter head.