Slip purlwise, yarn back or front?

I’m knitting two at a time, toe up socks. When working the heel bottom the instruction says to slip purlwise. By convention, is the yarn in the front or the back when not specifically stated?

If you are on the RS,then you would hold the yarn in back, unless visible threads are to be part of your design/pattern. In which case the pattern should say to hold the yarn in front.

The exact opposite is true from the WS.

KNITFreedom - Slip-Stitch Heel Pattern And Tutorial shows how to do it and what it should look like.