 Normally, these axes are represented by the letters X, Y and Z in order to compare them with some reference frame, usually named x, y, z.Normally, this is made in such a way that the X is used for the longitudinal axis, but there are other possibilities to do it.The yaw axis has its origin at the center of gravity and is directed towards the bottom of the aircraft, perpendicular to the wings and to the fuselage reference line.