Whipped stem stitch creates a raised heavier line than ordinary stem stitch. It is useful if you need to create a bold line particularly on curves.
A foundation row of stem stitch is first worked. The second thread is whipped over the line without picking up any ground fabric.
A contrasting colour thread can be used for the whipping. Use a blunt-ended tapestry needle for the whipping to avoid splitting the stitches on the foundation row.