What is an ssk?

Answer 1

It's a left-leaning decrease. Here are the steps:

1. slip a stitch knit-wise from the left needle to the right

2. slip a second stitch knit-wise from the left needle to the right

3. slip both of the stitches you just slipped back to the left needle and knit them together through the back loops.

Each stitch you slip knit-wise is twisted. When you knit through the back loop you twist them back.

Why go through all these gyrations instead of just knitting them together? A K2TOG creates a right-leaning decrease. A SSK creates a left-leaning decrease. When you do shaping, say of a sweater's shoulders, using paired decreases makes each edge of the shoulder a mirror image of the other. It gives garments a more finished, more tailored look.