One of the most time consuming endeavors when programming moving lights is shutter control. To achieve desirable effects, the shutter mechanisms need a lot of motors, and hence, a lot of control channels. Typically, most shutter assemblies have nine motors. There are four shutters, each using two motors to control its position within the aperture of the fixture and a ninth to rotate the entire assembly clockwise or counterclockwise. Many times these channels are labeled like this:
The oval in the image above shows how the light would fall on stage if the fixture was hung in a typical Front of House position. You can imagine that trying to make shutter cuts can be a time consuming effort of hunting and pecking for the right channel or more likely, pair of channels. That is why Natural Language Control groups related pairs together into useful names like Top, Bottom, Left and Right.
In the example shown below, the console is driving a Vari*Lite 1000 fixture. Cognito conveniently places the thrust controls on the two wheels closest to you (red and green) and the respective angle controls above those. One wheel manipulates motors 1a and 1b in unison to Thrust the shutter into the aperture of the fixture. The yellow wheel above that adjusts the relationship between those two motors, giving you one handle for controlling the Angle of that shutter. If you rotate the yellow wheel to the right, the shutter rotates right:
Note that the Thrust is measured in percentage. Most moving lights only allow you to put the shutter part way into the field of light. Above you can see the Top Thrust for the VL1000TSD is set to 25%, but its maximum value is 80% (not 100%). That is because the VL1000 physically can only cut out 80% of the beam. The angle is limited to -35° to 35°. Conversely, the VL3500 can only push the shutters 47% of the way into the beam. By using a percentage unit to describe the position in the aperture and degree units to describe the angle, you can copy the shape of a shutter cut from one type of moving light with one set of physical constraints to another with predictable results.
In the example below, the value of Top Thrust is set to 50% and the Top shutter is cutting the beam in half. The Right Thrust cuts 15% of the beam.
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