In ITF Taekwondo, movements 15, 16, 18, and 19 of Yulgok are hooking blocks, as far as I know.
Recently, however, I've been practicing Taekwondo with people who learned in different dojos, and a couple of them were taught to use knife-hand blocks at this point in the pattern instead.
Is this usual in an alternate ITF off-shoot or some other Taekwondo tradition I'm unfamiliar with?