Page "Göktürks" Paragraph 4

They were known in historical Chinese sources as 突厥 ( Pinyin: Tūjué, Wade-Giles: T ' u-chüeh, Middle Chinese Guangyun: dʰuət-kĭwɐt ).

According to Chinese sources, the meaning of the word Tūjué was " combat helmet " ( 兜鍪 ; Pinyin: dōumóu, Wade-Giles: tou-mou ), reportedly because the shape of the Jinshan ( 金山 jīnshān, Altai Mountains ), where they lived, was similar to a combat helmet-hence they called themselves 突厥 ( Tūjué / T ' u-chüeh ).