Cov khoom seem uas ntev dua uas muab tais tau ua kom lub thawv tais tau 180 degrees ntawm kab thawj thiab 135 degrees ntawm kab thib ob, txhawm rau qhib lub thawv tau yooj yim thaum pub cov tshuaj rau hauv, kev hloov pauv yooj yim ntawm cov khoom muab kev yooj yim heev rau kev teeb tsa cov khoom siv rau lwm lub thawv qauv.