Izhitsa stands forty third and last in the Early Cyrillic alphabet. It is Greek upsilon, and it is read as i or as v according to the letters around it, following the Byzantine pronunciation of the Greek digraphs.
Its name is unique in the set. It is a diminutive formed on the name of another letter, izhe, so it means something like the little i, and it is the only Slavonic letter name built out of a Slavonic letter name rather than out of a word of the language or a Greek original.
Its numeral value is 400 in the printed Church Slavonic tables, which follow the Greek series exactly. The manuscript tradition gives that value to uk instead, and the two usages cannot be reconciled; a reader working with a number in the four hundreds needs to know which tradition the book follows.
Like ksi, psi and fita it exists for Greek spelling and for the numeral series, and it writes nothing Slavonic requires. Its use narrowed steadily over the centuries to a handful of Greek derived words in the church books, and its final decline is the subject of the companion article.
It is worth noting simply as a matter of order that the Slavonic alphabet ends on a Greek letter, as it begins on a Greek shape carrying a Slavonic name.