A Karate stance
very similar to the Yoko Seishan dachi
except narrower. To form Sanchin dachi start with feet shoulder width apart, then move one foot in by about the width of your foot (NB not by a foot
as in 12 inches, more like 3 inches). Now turn both your feet inward slightly (about 15°). Finally imagine a line running directly behind your heels, perpendicular to the direction you are facing. Step one foot so that the toes are just behind this line, and you should now be in Sanchin dachi. Weight distribution should be even and your upper body should be facing straight ahead (forward facing). Bend knees slightly to relieve stress on joints.
This is used for blocks etc, in a similar fashion to other narrow stances.