Page "CMOS" Paragraph 17

More complex logic functions such as those involving AND and OR gates require manipulating the paths between gates to represent the logic.

When a path consists of two transistors in series, both transistors must have low resistance to the corresponding supply voltage, modelling an AND.

When a path consists of two transistors in parallel, either one or both of the transistors must have low resistance to connect the supply voltage to the output, modelling an OR.