Purl Right Side?

[meredith123]

I am new to knitting, so forgive me if this is obvious to most of you. I have instructions that say for a particular row to "Purl on RS, Knit on WS". Doesn't one usually knit the RS and Purl the WS? If so, why would a pattern tell me to do the opposite?

Thanks

[suzeeq]

Because this is a reverse stockinette stitch? Or if only for one row there's a purl ridge that shows on the RS.