Sl2-k1-psso - Slip 2 stitches together, knit 1, pass slipped stitches over the k2tog (can also be abbreviated as S2kp)
Worked on the knit side, this double decrease will reduce the overall stitch count by 2 stitches.
It will create a straight centre stitch with the stitches each side leaning towards the centre
1. Insert the right hand needle into the 2nd and 1st stitches, from right to left, and slip both stitches together to the right hand needle
2. Knit the next stitch on the left hand needle
3. Insert the left hand needle into the front of the 2 slipped stitches and lift these over the knit stitch