Claim: In the Cartesian plane, two vectors are said to be perpendicular if the angle between them is 90°, while in Cartesian space, the norm of a vector is the square root of the vector dotted with itself.
Evidence: In Cartesian space, the norm of a vector is the square root of the vector dotted with itself. In the Cartesian plane, two vectors are said to be perpendicular if the angle between them is 90° (i.e. if they form a right angle).