Sha stands twenty seventh and writes sh, the voiceless palato-alveolar fricative. It carries no numeral value, because Greek had no letter for the sound and therefore no number attached to any sign that could be borrowed for it.
Among the letters supplied for Slavonic the sha is the least troubled. It has no doublet, so there are no distribution rules to learn; its sound never merged with anything, so no later reform found it redundant; and it appears in every modern Cyrillic alphabet in a shape essentially unchanged from the earliest manuscripts. Very few letters in this alphabet have had so uneventful a history.
It is also structurally productive. The letter for the cluster sht is built on it, whether as a ligature made inside Cyrillic or as a Glagolitic sign redrawn to match, and the resemblance between the two is visible at a glance.
The letterform is symmetrical about a vertical axis, which is rare in this script and has attracted comment from everyone who has compared it with other alphabets. That symmetry is the main reason it is the letter most often produced in the argument that some Cyrillic signs came from a Semitic source, and the argument is taken up in the companion article.