I need to seam a side on a piece I was doing all knit stitch on so I was going to use the mattress stitch but she shows it on stockinette stitch with the V’s. Can I still do it on all knit stitch???
Yes, I believe it is still called mattress stitch. Here is a link to a page from a free on-line knitting book that has a tutorial on how to do mattress stitch on garter stitch (knit every row). LINK