Theory of Machine
The Grubler's criterion for determining the degrees of freedom (n) of a mechanism having plane motion is (where l = Number of links, and j = Number of binary joints)

n = (l -1) - j
n = 2(l - 1) - 2j
n = 3(l - 1) - 2j
n = 4(l - 1) - 3j

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Theory of Machine
Open pairs are those which have

Point or line contact between the two elements when in motion
Two elements that permit relative motion
Elements of pairs not held together mechanically
Surface contact between the two elements when in motion

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