Data on more than 6000 joints cutting the nearly horizontal Paleozoic strata of east, central, and southern New York and northern Pennsylvania reveal three contrasting regional sets. The local consistency of the joint pattern and a gradual regional swing are established. The lack of consistent relationships of the joints to other structures, especially the tracing of a “dip” set across the area into the “strike” position, indicates that the joints formed independently of, and earlier than, the folds, faults, and regional dip. The principal set consists of two conjugate shears intersecting at very acute angles (averaging 19°); its pattern is radiating for the whole area. Theoretical analysis, confirmed by experimental tests, points to an origin by shearing under simultaneous compression and tension acting at right angles. The second set is single and lies about perpendicular to the first, thus forming a pattern of concentric arcs; it originated from tension. These two sets constitute a system of practically contemporaneous fractures. The minor set mantains a constant position for the entire area; it probably is tensional but originated under doubtful circumstances, perhaps at a different time.