Trapezoidal thav duab
Nws muaj li ntawm ob qhov ntev ua ke thiab ob peb nqaj teeb. Cov kab uas hla ntawm qhov ntev ntawm kab teeb yog trough, zigzag, lub thawv, thiab lwm yam. Cov kab hlau txuas tau ua rau nkhaus, tsis tu ncua-ntu lossis cov nce mus nce los-hauv cov dav lossis kab rov tav.
Tus ncej trapezoidal yog dav siv rau cov tsheb thauj khoom thiab qee cov tsheb thauj neeg vim tias nws lub siab khoov siab zog thiab ceev ceev thiab yooj yim qhov chaw nruab. Nws tsis tshua muaj siv hauv cov tsheb thauj neeg.