A tag (Aramaic: תאג, plural tagin, תאגין) is a decoration drawn over some Hebrew letters in the Jewish scrolls of Sifrei Kodesh, Tefillin and Mezuzot. The Hebrew name for this scribal feature is kether (כתר). Tag and kether mean 'crown' in Aramaic and Hebrew respectively.
In modern practice, the letters Beth, Daleth, He, Heth, Yud and Quf have one tag (Mnemonic: BeDeQ-ChaYaH בדק חיה). The letters Gimel, Zayin, Tet, Nun, Ayin, Tzadi and Shin, as far back as Talmudic times, have 3 tags (Mnemonic: Sha´ATNeZ-GaTz שעטנז גץ). Some manuscripts feature embellished tagin on the top line of each column and some also on all occurrences of the Tetragrammaton other than those prefixed with a lamed.
About the 2nd century CE, a work called Sefer Tagin (ספר תאגין or ספר תאגי) emerged attributed to Rabbi Akiva which laid out the 1960 places where modified tagin or letter forms occur in a Torah scroll. In it, the locations of letters which receive a number of tagin which differs from the sha'atnez gatz tradition, e.g. the initial beit of bereshith in Genesis 1:1 having 4 tagin as opposed to the usual 1 and the instances of aleph which bear 7 tagin apiece. According to this work, each occurrence of each letter is to be written with between 0 and 7 tagin, as delineated in the lists contained therein.
This tradition, predating the versification of the Torah text, contains some instruction wherein it is difficult to know what verses are being referenced, thus in the 12th century, Maimonides ruled that though a scribe should do his utmost to incorporate all of the elements of this tradition, called otiyyot meshunot (strange letters), if they are omitted, whether in full or in part, the scroll would not be ruled as pasul (invalid).
The Talmud states that tagin was originally added to the text by God at Mount Sinai, and that Rabbi Akiva would use their presence in order to derive laws.
In kabbalistic thought, each tag has special significance and meaning.