Shikii (敷居) – the lower horizontal frame for Japanese shoji and fusuma doors. By custom, the shikii is to be stepped over and never on. The grooves routed out in the shikii are relatively shallow when compared to the kamoi. And as with the kamoi, the term shikii is also used loosely for the same horizontal frame component in modern sliding door-rail systems as well.
Notice the plastic strips lining the tracks to limit friction when sliding the door back and forth.