Single-closed-loop N-bar mobility criteria are presented. These criteria provide a simple and effective method (1) to determine the full rotatability of any single closed-loop linkages, (2) to predict the revolvability between two adjacent links, (3) to explain and identify the existence of singular positions, (4) to classify linkages, and (5) to identify the difference of singular positions between linkages of different types. These criteria are fundamental tools toward a better understanding on the mobility of more complicated linkages.