Some of these forms are very rare and used sparingly. In particular, the (-"ke") / (-"shike") are used almost exclusively in an ancient construction called ("Ku-gohō" "Ku-grammar") which uses the irrealis form to form nouns from verbs and adjectives; e.g., ("yasu-shi" "peaceful") → ("yasu-ke") + (-"ku") → ("yasukeku" "peace of mind"). The construction (-"kuba") / (-"shikuba") appears to be an irrealis form (-"ku") / (-"shiku") + particle (-"ba") (since that particle usually attaches to the irrealis form), but is actually (-"ku") / (-"shiku") + particle ("ha"; modern pronunciation "wa") with a sequential voicing sound change from ("ha") to ("ba").