Right-hand rule: The trick for right-handed coordinate systems to establish which way the cross product of two three-vectors will be directed. There are some forms of this rule, and it can be exerted in many manners. If u and v are two vectors that are not parallel, then u cross v is a vector that is directed in the following way: Orient your right hand and therefore your thumb is perpendicular to the plane stated by the vectors u and v. If you can twist your fingers in the direction from vector u to v, your thumb will position in the direction of u cross v. (When it does not, the vector is directed in the opposite direction.) This has instant application for recognizing the orientation of the z-axis basis unit vector, k, in terms of the x- and y-axis basis unit vectors; twist your right hand in the direction of i to j, and your thumb will point in the direction of i cross j = k.
The rule is too applicable in numerous practical applications, like determining which way to turn a screw, and so forth. There is as well a left-hand rule that shows opposite chirality.