iSTd

The iSTd scale is a combined scale, only
available on the slide in position 2, or 3, and that
combines four differential trigonometric scales into one
scale. The four scales represented are:
Sd  used for sine trigonometric
calculations with values from 0.0° to 90.0°
Td  used for tangent
trigonometric calculations with values from 60.0° to 0.0°
ISd 
used for inverse sine trigonometric calculations with
values from 1.0 down to 0.0.
ITd  used for inverse tangent trigonometric
calculations with values from 0.0 to 1.76.
These scales work in conjunction with the D scale and the
indexes of that scale to perform more complex differential
sine, tangent, arcsine and arctangent calculations.
