The Keethera was, as approximately translated from Vulcan, a "structure of harmony." Constructing a keethera was a Vulcan meditation technique requiring precise balance and spatial acuity. It helped to focus thought and refine mental control. The structure, involving a group of building blocks, was different each time and dependent on the state of mind of the builder.
The following meditation was recited during the building of a keethera: "Structure. Logic. Function. Control. The structure cannot stand without a foundation. Logic is the foundation of function. Function is the essence of control. I am in control. I am in control." (VOY: "Flashback")