Sentence:  And the bed that sagged in a <WSD>certain</WSD> place where all the weight had been put too many times before and the walls fine and thin for overhearing talk in the next room when Gratt went out for ice , the sound coming through the walls like something on the other side of the curtain , so you knew they heard you when they were quiet and while you lay wondering what they had heard you listened . 
Meaning of the word marked with <WSD> tags:  reliable in operation or effect