As the shogunate was supposedly the household manager of the shogun family, there was such a strictly abided unwritten rule that all important posts of the shogunate went to fudai daimyo, except for the cases of the bakumatsu period (the last days of the shogunate) and the Ochi-Matsudaira family.
幕府は将軍家の家政機関であると言う建て前上、幕閣の要職には、幕末及び越智松平家の例外を除き譜代大名以外からは、登用しない慣行が不文律として厳格に守られた。