Eventually so many rules sprang up, as each regiment improvised their own variations, two versions came into use. One, known as "rigid Kriegspiel", was played by strict adherence to the lengthy rule book. The other, "free Kriegspiel", was governed by the decisions of human umpires. Each version had its advantages and disadvantages: rigid Kriegspiel contained rules covering most situations, and the rules were derived from historical battles where those same situations had occurred, making the simulation verifiable and rooted in observable data, which some later American models discarded. However, its prescriptive nature acted against any impulse of the participants towards free and creative thinking. Conversely, free Kriegspiel could encourage this type of thinking, as its rules were open to interpretation by umpires and could be adapted during operation. This very interpretation, though, tended to negate the verifiable nature of the simulation, as different umpires might well adjudge the same situation in different ways, especially where there was a lack of historical precedent. In addition, it allowed umpires to weight the outcome, consciously or otherwise.