A vertical joint system for a substrate having an opposed major first and second surfaces, the joint system comprising: male and female joints (Jm, Jf) being non-symmetrical joints extending along opposite sides of the substrate, the male and female joints configured to enable two substrates with like joint systems to engage each other in response to a force applied in an engagement direction which is perpendicular to the major surfaces; the male and female joints each provided with two laterally spaced transversely extending surfaces (Cm1, Cm2; Cf1, Cf2) configured to enable the male joint of a first substrate to engage the female joint of a second substrate with the first and second transversely extending surfaces of the female joint (Cf1, Cf2) located relative to the first and second transversely extending surfaces of the male joint (Cm1, Cm2) to form respective first and second locking planes (18, 20) on an innermost and an outermost side of each joint, each locking plane lying parallel to the engagement direction and wherein the transversely extending surfaces associated with each locking plane extend laterally toward each other from opposite sides of the locking plane with the transversely extending surfaces of the female joint (Jf) overhanging the transversely extending surfaces of the male joint (Jm) to inhibit separation of the engaged joints, wherein in at least one of the transversely extending surfaces associated with each locking plane has a curved profile, wherein at least one of the transversely extending surfaces associated with at least one of the locking planes has a profile of a continuous convex curve, characterized in that the transversely extending surfaces are configured to enable relative rotation of two engaged substrates by up to 3° while maintaining engagement of the two substrates, wherein the male and female joints are configured such that a +3° rotation of the first substrate relative the second substrate causes the male projection (Pm) to rotate within the female recess (Rf) and the second male transversely extending surface to ride or roll up, but not past an apex of, the second female transversely extending surface. male and female joints (Jm, Jf) being non-symmetrical joints extending along opposite sides of the substrate, the male and female joints configured to enable two substrates with like joint systems to engage each other in response to a force applied in an engagement direction which is perpendicular to the major surfaces; the male and female joints each provided with two laterally spaced transversely extending surfaces (Cm1, Cm2; Cf1, Cf2) configured to enable the male joint of a first substrate to engage the female joint of a second substrate with the first and second transversely extending surfaces of the female joint (Cf1, Cf2) located relative to the first and second transversely extending surfaces of the male joint (Cm1, Cm2) to form respective first and second locking planes (18, 20) on an innermost and an outermost side of each joint, each locking plane lying parallel to the engagement direction and wherein the transversely extending surfaces associated with each locking plane extend laterally toward each other from opposite sides of the locking plane with the transversely extending surfaces of the female joint (Jf) overhanging the transversely extending surfaces of the male joint (Jm) to inhibit separation of the engaged joints, wherein in at least one of the transversely extending surfaces associated with each locking plane has a curved profile, wherein at least one of the transversely extending surfaces associated with at least one of the locking planes has a profile of a continuous convex curve, characterized in that the transversely extending surfaces are configured to enable relative rotation of two engaged substrates by up to 3° while maintaining engagement of the two substrates, wherein the male and female joints are configured such that a +3° rotation of the first substrate relative the second substrate causes the male projection (Pm) to rotate within the female recess (Rf) and the second male transversely extending surface to ride or roll up, but not past an apex of, the second female transversely extending surface.