Necessary conditions draw category boundaries while family resemblance structures organize what lies inside.

Evidence

If one is looking for a template for gray zone concepts, equation (7.1) could be it. Roughly, one uses necessary conditions to work on the boundary delimitation problem and then within the boundaries one uses family resemblance structures. — Gary Goertz, Social Science Concepts and Measurement, 1150

Categorization

• DEEPTHINKING: A compact design recipe — ANDs solve the outer-boundary problem, ORs handle internal gradation — portable to any fuzzy category from democracy to mental illness to species.
• SEED: Angle: ‘Every fuzzy category has the same skeleton’ — necessary-and-sufficient boundaries with family-resemblance interiors, from DSM diagnoses to democracy indices.