Split stitch
Kensington outline stitch
This is a form of back stitch, with the needle splitting the centre of the previous stitch rather than coming up in the same hole.  This results in what looks like a mini chain stitch.
The earliest notable use of split stitch is within Opus Anglicanum where it was used extensively as a filling stitch to provide fine details such as faces.  It was used in a similar manner in 14th century Byzantine embroidery. 
It continued to be used in traditions across the world: in late/post Medieval Icelandic (especially ecclesiastical) embroidery, split stitch was one of a small group of prominent stitches; in China during the Ming dynasty it was used to create precise lines; 17th century floral Icelandic embroidery used to decorate garments was predominantly worked in split stitch; 19th century Arts and Crafts embroidery used split stitch as filling stitch for flesh.
