© 1999 Springer-Verlag Berlin Heidelberg. The use of Go terms while playing Go differs according to the player’s skill. We conduct three experiments to examine this in detail. In the first experiment, players’ spontaneous utterances (called protocols) were collected. We analyze these protocols in two ways. One is the number of Go terms used, and the other is the contents of the terms, such as strategic or tactical. The second experiment examines how well the players knew the configurations of the stones. From the two experiments, we find that even if the subjects know of many Go terms, their use depends on the subject’s skill. The third experiment considers “Soudan- Go,” where two players form a team. They are in the same room and can freely talk to each other; their spontaneous utterances (protocols) were collected. We also analyze reports of “Houchi Soudan-Go,” which is a Soudan-Go match between professional players. We find that expert players often use Go terms and they understood their partner’s intentions without needing a full explanation. Intermediate level players often talked over their plan and their opponent’s plan using many Go terms. From our analyses we developed a hypothesis which we call the iceberg model. The purpose of the model is to explain the structure of a term in the human brain from the viewpoint of the role of the term. Although this is still a hypothesis, it will become an important guide when carrying out protocol analyses and modeling the thought processes of Go players.