The Yardstick (ものさし, Monosashi) is an item in the first Suikoden. It can be found in Kirov and its only use is to trade it for Salt as part of the fetch quest to recruit Sarah.
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When McDohl attempted to recruit Sarah in Kirov by getting her some Soap, he had to embark on a series of trades in the city. As part of this, McDohl traded the Sugar he bought in the Kirov item shop for the yardstick which, in turn, he traded for Salt.