Context: In heartwood it occurs only in the first and last forms. Wood that is thoroughly air-dried retains 8–16% of the water in the cell walls, and none, or practically none, in the other forms. Even oven-dried wood retains a small percentage of moisture, but for all except chemical purposes, may be considered absolutely dry.

Question: What does wood retain in its cell walls despite being air-dried?
Answer: water

Question: Air-dried wood keeps up to what percentage of the water in its cell walls?
Answer: 16

Question: What kind of dried wood retains a small quantity of water but is considered absolutely dry?
Answer: oven-dried