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Normally potentiometers have three pins, but the Song Huei's SL-20V has four. According to the datasheet: 1, 2, 2', and 3

What is the fourth pin for?

Update

Disassembly shows that @jippie's assumption is correct. Pins 2 and 2' are interconnected, and there are indeed two sliders (barely visible on my photo):

Disassembled SL-20V

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    \$\begingroup\$ Probably, 2 and 2' are electrically connected inside the pot. The datasheet should spell such things out, but it doesn't seem to in the case of this pot. \$\endgroup\$ – Nick Alexeev Aug 5 '13 at 6:27
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2 and 2' are probably internally interconnected, both to the slider. If you open one of those potmeters you'll find that there are actually two sliders, one on the resistor itself and one on a brass or gold plated conductor in parallel to the resistor.

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    \$\begingroup\$ Also, an easy test would be to slide the tab to around one-third, then measure resistances between all the pin combinations. Most likely, 2 and 2' would show 0 Ohms between them. \$\endgroup\$ – Anindo Ghosh Aug 5 '13 at 6:42

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