糸（jiao）绞丝底（jiao si di ）different dictionaries list the same character as having a different radical “甥”，“舅”二字，《说文解字》根据六书体系，都归“男”部；《康熙字典》则依检字法原则，以“甥”入“生”部，“舅”入“臼”部。
Not a few words can explain the clear.
i dont know if you can read chinese .if you can ,then go to this website. http://baike.baidu.com/view/277746.htm
when i in primary school,teacher told me how to use dictionaries,if a word have two radicals.normally try the easy simple one,if cant find the word,then another.so if you let me to find 累 in dictionary,i will try 田 first.
hope my answer can help you,