In its archaic form, θ was written as a cross within a circle, and later, as a line or point in circle. Archaic crossed forms of theta are seen in the wheel letters of Linear A and Linear B. The cursive form ϑ was retained by Unicode as U+03D1 ϑ "GREEK THETA SYMBOL", separate from U+03B8 θ "GREEK SMALL LETTER THETA". For the purpose of writing Greek text, the two can be font variants of a single character, but θ and ϑ are also used as distinct symbols in technical and mathematical contexts.
Latin
In Latin script used for the Gaulish language, theta developed into the tau gallicum, conventionally transliterated as Ð, although the bar extends across the centre of the letter. The phonetic value of the tau gallicum is thought to have been. In March, 2020 Unicode added four characters to represent tau gallicum:
A Bayer designation letter applied to a star in a constellation; usually the eighth star so labelled but not necessarily the eighth-brightest as viewed from Earth
One of the variables known as "Greeks" in finance, representing time decay of options or the change in the intrinsic value of an option divided by the number of days until the option expires
A variable indicating temperature difference in heat transfer
Measuring the angle of incident X-ray beam during XRD
According to Porphyry of Tyros, the Egyptians used an X within a circle as a symbol of the soul; having a value of nine, it was used as a symbol for Ennead. Johannes Lydus says that the Egyptians used a symbol for Kosmos in the form of theta, with a fiery circle representing the world, and a snake spanning the middle representing Agathos Daimon. The Egyptians also used the symbol of a point within a circle to represent the sun, which might be a possible origin of its use as the Sun's astrological glyph. It is worthwhile to note that θῆτα has the same numerical value in isopsephy as Ηλιος : 318.
Abbreviation
In classical Athens, it was used as an abbreviation for the Greek θάνατος and as it vaguely resembles a human skull, theta was used as a warning symbol of death, in the same way that skull and crossbones are used in modern times. It survives on potsherds used by Athenians when voting for the death penalty. Petrus de Dacia in a document from 1291 relates the idea that theta was used to brand criminals as empty ciphers, and the branding rod was affixed to the crossbar spanning the circle. For this reason, use of the number theta was sometimes avoided where the connotation was felt to be unlucky—the mint marks of some Late Imperial Roman coins famously have the sum ΔΕ or ΕΔ substituted as a euphemism where a Θ would otherwise be expected.
Character encodings
Greek Theta
Coptic Thethe
Cyrillic Fita
Mathematical Theta
These characters are used only as mathematical symbols. Stylized Greek text should be encoded using the normal Greek letters, with markup and formatting to indicate text style.
