The hiragana syllable く(ku). Its equivalent in katakana is ク(ku). It is the eighth syllable in the gojūon order; its position is か行う段(ka-gyō u-dan, “row ka, section u”).
Usage notes
Do not confuse it with the kanaiteration mark〱 which is sometimes used in vertical text.
Originally a compound of the 連体形(rentaikei) of verbs and adjectives and 所(aku), 上代特殊仮名遣い(jōdai tokushu kanazukai) contractions of vowel clusters caused it to be reanalyzed as the suffix く(ku) following the 未然形(mizenkei) of 四段活用(yodan katsuyō) and ラ行変格活用(ra-gyō henkaku katsuyō) verbs, the suffix らく(raku) following the 終止形(shūshikei) of 二段活用(nidan katsuyō), サ行変格活用(sa-gyō henkaku katsuyō), カ行変格活用(ka-gyō henkaku katsuyō), and ナ行変格活用(na-gyō henkaku katsuyō) verbs - as well as following the 未然形(mizenkei) or 連用形(ren'yōkei) of 一段活用(ichidan katsuyō) verbs, and the suffix けく(keku) following the stem of adjectives.
The use of this nominalizer is called ク語法(ku-gohō).