1. Horizontal differentiation: linear city
   1. Differentiated goods and price dispersion
   2. Linear city by Hotelling (1929)
2. Vertical differentiation: heterogeneous quality
   1. Different qualities
   2. Monopolist preferred quality
   3. Duoploy with vertical differentiated products
   4. Vertical differentiation analysis
3. Horizontal differentiation with entry: circular city
   1. Salop (1979)