Tsaadiy is the eighteenth letter of the Samaritan alphabet and carries the numeral value ninety, the last of the tens. It stands for what was an emphatic s in the older language, the second member of the emphatic series after tit. The name is the inherited Northwest Semitic one behind Hebrew tsade.
This letter absorbed more of the ancestor language than almost any other. Several distinct emphatic consonants of the earlier system fell together in the languages that used the twenty-two letter alphabet, and all of them are written here. Arabic, which fixed its alphabet for a language that had kept the distinctions, writes the same territory with more than one letter, which is why the relationship between an Arabic word and its Hebrew or Samaritan cousin containing this letter can be perfectly regular and still look unrecognisable.
In the square Hebrew script this letter has a separate final form. Samaritan writes one shape everywhere.
Greek inherited a sign in this position, called san, and did not keep it. San competed with sigma for the same job, lost, and dropped out of the alphabet early, which is why the Greek sequence has no counterpart to tsaadiy and why the correspondence between the Samaritan and Greek alphabets, exact almost everywhere else, breaks off here and again at quf.
The character is encoded at U+0811.