The Y-axis is also known as growth axis because
Y-axis
It is also called as growth-axis angle, measured as the posterior inferior angle on S–Gn–FHP. The Y-axis indicates the degree of downward and forward position of the chin in relation to the upper face. A large Y-axis is indicative of the clockwise rotation of the mandible. Y-axis does not give an indication of the size of the lower jaw but the direction in which it is growing. Mean 59.4°, range 53°–66°.
Its axis is parallel to patient's growth in height
It is an indicator of the direction of growth pattern
It is an indicator of the amount of facial growth
It increases in size as the growth increases