coordinate system
frame of reference
reference system
reference frame

His index of fables has been used as a reference system by later authors.

First, he had to establish some kind of reference system.

According to his own reference system, this inaugurated a "green period".

One such reference system is a fixed quantity of gas.

Another problem with the reference system is that caustics can occur which cause the gauge choice to break down.

Place names provide the most useful geographical reference system in the world.

Whenever at rest, it was so always with regard to some given reference system, but not necessarily to another.

Then find the corresponding spoke on the hexaxial reference system.

Calibration must also be done using a reference system in order to ensure accuracy.

It is altogether possible that they 1 Directional reference system.