It's a very complicated question. 干 in simplified Chinese mainly stems from three traditional Chinese characters which are 干, 幹 and 乾. 1. If 干 in simplified Chinese means ①shield, ②river bank, ③offend or ④to involve, its traditional form is 干(with pinyin gan in the 1st tone). 2.If 干 in simplified Chinese means ①trunk, ②main part of things or ③to do, its traditional form is 幹 (with pinyin gan in the falling tone). 3.If 干 in simplified Chinese means ①dry, ②empty or ③a kinship but not blood ties, its traditional form is 乾(with pinyin gan in the 1st tone). ps: in some time 乾 also has the meaning of "to do" and its pronunciation is gan in the falling tone. It is a special case that there is no simplified form of 乾 if it means something about the Eight Trigrams(in this case its pronunciation is qian in the 2nd tone ). There are some other traditional forms of 干. Such as 榦, 乹, 亁(this is not 乾...they are slightly different in character) ... In short, it is quite intricate...