The SPIN model consists of four types of questions used in sales: Situation, Problem, Implication, and Need-payoff questions. Situation questions gather facts about the customer. Problem questions identify the customer's problems and needs. Implication questions explore the effects of the problems. Need-payoff questions develop the customer's desire for a solution. The model was developed by Neil Rackham and aims to understand customer problems in order to provide appropriate solutions that customers will want to buy.