Kako stands twelfth, writes k, and carries the numeral 20 from Greek kappa. Its name is the Slavonic interrogative meaning how, one more ordinary word of the language pressed into service as a letter name.
The shape is the uncial kappa and it has changed very little in eleven hundred years, which makes it one of the most stable signs in the alphabet. Its numeral value opens the tens series, so it does constant work in the numbering of psalms, of the days of a month and of the chapters of a Gospel, always under a titlo.
The sound it writes was involved in one of the defining sound changes of Slavic. Before certain front vowels the inherited k had already shifted, in the prehistory of the language, to ch and to ts, which is why Slavonic has three letters where an observer might expect one, and why the alternation between them runs through the grammar of every Slavic language today. The alphabet inherited the result rather than the process, and gave each of the three outcomes its own sign.
Kako survives in every modern Cyrillic alphabet without alteration. It is also one of the letters most often used as an abbreviation mark in liturgical books, standing with a titlo for a whole word in rubrics and marginal notes, a practice that assumes a reader who already knows the text well enough to expand it.