Smyrna Cross is the same stitch as a double cross stitch. It can be worked over any number of threads in one or more colors. In these examples I’ve shown the stitch worked over 2 threads and over 4 threads.
To work the Smyrna cross, work the diagonal cross first, followed by an upright cross. The intersection of the stitch is 4 threads high, which gives this stitch some height and definition. Use it to till an area, individually as an accent, or in bands and rows.