Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
dĭăgōnālis, e, adj. [from διαγώνιος and the Lat. ending alis), diagonal: linea, Vitr. 9, 1; 6, 5.
And subst., dĭăgōnĭum, ii, n.: diagonii linea, id. 6, 4.