Trig ScalesTrig scales were in use almost as early as the logarithmic scales. Trig scales went through various inventions and improvements. Let us review the progress.

Early slide rules were so called "Mannheim" type, which had A =B, C= D scales on front side and =S(sin), L(log), T(tan)= on rear of the slide. S scale is referenced to A/B scales and graduated in degree/ minutes. Since A scale has two cycles, sin covers a wide range of 35 minutes to 90 degrees. T scale is referenced at C/D scale and covers 6 degrees to 45 degrees.
Exhibit 1. Trig sacles for K&E 40533 Why this asymmetry? One explanation is that, when the slide is reversed so that trig scales are on the front side, S and T scales are opposit A and D scales, respectively. This enables direct reading of trig values on A or D scales.
Besides, scales used for calculations were mainly A and B scals in early days. A/B scales were prefered bacause "over scales" happen less frequently. 
There is yet another variant to the Hemmi's SI/TI scales. It is on a Hemmi's Wattime economy rule and the earlier model of 2664. The scales are designated as SI1, SI2, TI1 and TI2.
Exhibit 5. Hemmi SI1, SI2, TI1, TI2 scales (Click above to view full range image. ) SI1 and TI1 are the regular inverted trig scales covering 6 degrees and up. SI2 and TI2 cover angles less than 6 degrees, very much the same way the ST scales do. But why two saparate scales? To get more accuracy on values near 6 degrees? May be. Sin and Tangent values at 5' 45" are about 0.5% apart. There may have been applications requiring better than that. And there is another neat idea built into TI2 scale. The right half of TI2 scale, just about beyond 2 degees, is replaced with the tan scale for between 45 and 70 degrees. So tangent smaller than 2 degrees share the SI2 scale and you get extra range up to 70 degrees for tangent. 