Usha dunta leh, oo sidoo kale loo yaqaan ul dunta si buuxda u leh ama urka (ereyada inta badan loo isticmaalo is beddelka xaaladaha aan farsamada ahayn), waa ul bir ah oo silindrical ah oo leh dunyo joogto ah oogada sare. Caadi ahaan waxa loo isticmaalaa in lagu xidho, la xidho, ama lagu taageero qaybaha qaabdhismeedka.