Most common site for the initiation of otosclerosis:

(c) Fissula ante-fenestrum(Ref. Cummings, 6th ed., 2212)Though otosclerosis ultimately results in the fixation of the footplate and margins of stapes which is lying over the oval window, it actually gets initiated most commonly in the immature cartilaginous area of the bony labyrinth which lies anterior to the fenestra of oval window and hence called fissula ante fenestrum.Fissula post-fenestrum area of the bony labyrinth is also cartilaginous but otosclerosis is uncommon here.