Define the vectors x and y as the directed line segments from a$6$ to b$8,$ and a$6$ to c$7$ respectively on a chessboard. Other traditional knight moves such as a$6$ to c$5,$ and a$6$ to b$4,$ are impossible for your white knight due to weak knees. Hence your knight can only move in the x$,$x$,$ y or $$y directions.