Hitomezashi (also called 'One-Stitch Sashiko') is a form of sashiko stitching which originated in rural Yamagata Prefecture in northern Japan. Hitomezashi patterns are designed on a grid system and stitched with a running stitch, except the intersecting stitches in hitomezashi touch each other whereas they don't in 'ordinary' sashiko.
Patterns in hitomezashi may look complicated, but they are reasonably easy to stitch as all you need to do is mark a grid of squares on your fabric and then you're good to go! Using a simple running stitch, work across the fabric horizontally from one side to the other. Stitch all the horizontal lines first, then repeat for the vertical lines, then stitch any diagonal lines to complete the design.
Hitomezashi usually takes more thread than 'ordinary' sashiko but the result is a strong, plump fabric with a complex-looking design.