Complementarity, Intervening opportunity, and Transferability are three factors that are the basis for transportation and interaction, as stated by Ullman as his argument in “The role transportation and the basis for interaction (p. 868), however the system should be kept in consideration by the investigators lest lead they are led astray by assigning exclusive weight to only one of the factor in attempting to explain past interaction or in predicting interaction under changed conditions (p. 872).
Complementarity is a function both of natural and cultural areal differentiation and of areal differentiation based simply on the operation of economics (p. 868). It is the demand for the specific product in one place and the supply of that product in another place. No matter how they transport the low-valued cheap raw materials for steel from West Virginia to Chicago or ship finished products to great distances from Washington to forest areas of the South, or the flow of animals and products from Iowa to the complementary Industrial Belt and California, the basic concept is complementarity.
Intervening opportunity help to create interaction between distant complementary areas by making the construction of intermediate transport routes profitable and thus paying part of the cost of constructing a route to the more distant source. The fact that Florida attracting more amenity migrants than California (p. 869), is used by Ullman as his statement to explain the intervening opportunity.
Transferability is another factor required in an interaction system measured in terms of transfer and time costs (p. 869). Ullman puts forward a condition that if the distance between market and supply is too great and too costly to overcome, the interaction will not take place despite perfect complementarity and lack of intervening opportunity. They will go for alternatives, instead.
Yet, the matter of fact is the process of interaction links only certain areas, often in a quite specialized way, and leaves other areas relatively untouched.