Axis F

Each letter N, S, T, F is a Cognitive Axis: a diameter.

Axis F diameterFour cognitive diameters through a shared center. Extroverted ends above the x-axis, introverted ends below. Vertical line splits left and right. Vertical line splitting the quadrantsN diameterNeNiS diameterSeSiT diameterTeTiF diameterFeFiNSTF

Ends

Stacks that use F

Every valid stack uses each letter once.