Theta (uppercase Θ or ϴ, lowercase θ (which resembles digit 0 with horizontal line) or ϑ; Ancient Greek: θῆτα, thē̂ta, ; Modern: θήτα, thī́ta, ; UK , US ) is the eighth letter of the Greek alphabet, derived from the Phoenician letter Teth . In the system of Greek numerals it has a value of 9.
Contents

Greek 1

Latin 2

Cyrillic 3

International Phonetic Alphabet 4

Mathematics and science 5

Lower case 5.1

Upper case 5.2

In its archaic form 6

Symbolism 7

Character encodings 8

See also 9

Notes 10
Greek
The alphabet on a
black figure vessel, with a pointandcircle theta.
In Ancient Greek, θ represented the aspirated voiceless dental plosive /t̪ʰ/, but in Modern Greek it represents the voiceless dental fricative /θ/.
Forms
In its archaic form, θ was written as a cross within a circle ( or ), and later, as a line or point within a circle ( or ).
The form ϑ was retained by Unicode as U+03D1 ϑ "GREEK THETA SYMBOL" ("=script theta"), 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
The name ARAÐÐOVNA on a Gaulish tombstone.
In the 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 [t͡s].
Cyrillic
The early Cyrillic letter fita (Ѳ, ѳ) developed from θ.
International Phonetic Alphabet
In the International Phonetic Alphabet, [θ] represents the voiceless dental fricative, as in thick or thin. It does not represent the consonant in the, which is the voiced dental fricative.
Mathematics and science
Lower case
The lowercase letter θ is used as a symbol for:
Upper case
The uppercase letter Θ is used as a symbol for:
In its archaic form

In its archaic form () theta is used as a symbol for Earth.
Greek mathematics
The common usage of theta in mathematical problems began with the Greeks. They logically chose this Greek symbol since it was the next in their alphabet not yet used and was easily legible when recreating for multiple uses.
Archaic crossed forms of theta are seen in the wheel letters of Linear A, Linear B, and Egyptian hieroglyphics, the mathematical tensor product, exclusive disjunction, and direct sum operators, the identification symbol for the Earth (as already mentioned), and the Celtic cross. The circumpunct, or circled dot, is seen in a wide range of places.
Symbolism
θ (θάνατος, death) in a mosaic

In ancient times, Tau was used as a symbol for life or resurrection, whereas the eighth letter of the Greek alphabet, theta, was considered the symbol of death.
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 (literally: good spirit).^{[2]}
The Egyptians also used the symbol of a point within a circle (, the sun disc) 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 (theta) has the same numerical value in isopsephy as Ηλιος (Helios): 318.
Abbreviation
In classical Athens, it was used as an abbreviation for the Greek θάνατος (thanatos, “death”) 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.^{[2]} 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.^{[3]} 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 (delta and epsilon, that is 4 and 5) substituted as a euphemism where a Θ (9) would otherwise be expected.
Character encodings
Character

Θ

θ

ϑ

ϴ

ᶿ

Unicode name

GREEK CAPITAL LETTER THETA

GREEK SMALL LETTER THETA

GREEK THETA SYMBOL

GREEK CAPITAL THETA SYMBOL

MODIFIER LETTER SMALL THETA

Encodings

decimal

hex

decimal

hex

decimal

hex

decimal

hex

decimal

hex

Unicode

920

U+0398

952

U+03B8

977

U+03D1

1012

U+03F4

7615

U+1DBF

UTF8

206 152

CE 98

206 184

CE B8

207 145

CF 91

207 180

CF B4

225 182 191

E1 B6 BF

Numeric character reference

Θ

Θ

θ

θ

ϑ

ϑ

ϴ

ϴ

ᶿ

ᶿ

Named character reference

Θ

θ

ϑ



DOS Greek

135

87

159

9F







DOS Greek2

172

AC

226

E2







Windows 1253

200

C8

232

E8







TeX

\Theta

\theta

\vartheta



Character

Ⲑ

ⲑ

Unicode name

COPTIC CAPITAL LETTER THETHE

COPTIC SMALL LETTER THETHE

Encodings

decimal

hex

decimal

hex

Unicode

11408

U+2C90

11409

U+2C91

UTF8

226 178 144

E2 B2 90

226 178 145

E2 B2 91

Numeric character reference

Ⲑ

Ⲑ

ⲑ

ⲑ

Character

Ѳ

ѳ

Unicode name

CYRILLIC CAPITAL LETTER FITA

CYRILLIC SMALL LETTER FITA

Encodings

decimal

hex

decimal

hex

Unicode

1138

U+0472

1139

U+0473

UTF8

209 178

D1 B2

209 179

D1 B3

Numeric character reference

Ѳ

Ѳ

ѳ

ѳ

Character

횯

훉

횹

훝

Unicode name

MATHEMATICAL BOLD
CAPITAL THETA

MATHEMATICAL BOLD
SMALL THETA

MATHEMATICAL BOLD
CAPITAL THETA SYMBOL

MATHEMATICAL BOLD
THETA SYMBOL

Encodings

decimal

hex

decimal

hex

decimal

hex

decimal

hex

Unicode

120495

U+1D6AF

120521

U+1D6C9

120505

U+1D6B9

120541

U+1D6DD

UTF8

240 157 154 175

F0 9D 9A AF

240 157 155 137

F0 9D 9B 89

240 157 154 185

F0 9D 9A B9

240 157 155 157

F0 9D 9B 9D

UTF16

55349 57007

D835 DEAF

55349 57033

D835 DEC9

55349 57017

D835 DEB9

55349 57053

D835 DEDD

Numeric character reference

𝚯

𝚯

𝛉

𝛉

𝚹

𝚹

𝛝

𝛝

Character

훩

휃

훳

휗

Unicode name

MATHEMATICAL ITALIC
CAPITAL THETA

MATHEMATICAL ITALIC
SMALL THETA

MATHEMATICAL ITALIC
CAPITAL THETA SYMBOL

MATHEMATICAL ITALIC
THETA SYMBOL

Encodings

decimal

hex

decimal

hex

decimal

hex

decimal

hex

Unicode

120553

U+1D6E9

120579

U+1D703

120563

U+1D6F3

120599

U+1D717

UTF8

240 157 155 169

F0 9D 9B A9

240 157 156 131

F0 9D 9C 83

240 157 155 179

F0 9D 9B B3

240 157 156 151

F0 9D 9C 97

UTF16

55349 57065

D835 DEE9

55349 57091

D835 DF03

55349 57075

D835 DEF3

55349 57111

D835 DF17

Numeric character reference

𝛩

𝛩

𝜃

𝜃

𝛳

𝛳

𝜗

𝜗

Character

휣

휽

휭

흑

Unicode name

MATHEMATICAL BOLD ITALIC
CAPITAL THETA

MATHEMATICAL BOLD ITALIC
SMALL THETA

MATHEMATICAL BOLD ITALIC
CAPITAL THETA SYMBOL

MATHEMATICAL BOLD ITALIC
CAPITAL THETA SYMBOL

Encodings

decimal

hex

decimal

hex

decimal

hex

decimal

hex

Unicode

120611

U+1D723

120637

U+1D73D

120621

U+1D72D

120657

U+1D751

UTF8

240 157 156 163

F0 9D 9C A3

240 157 156 189

F0 9D 9C BD

240 157 156 173

F0 9D 9C AD

240 157 157 145

F0 9D 9D 91

UTF16

55349 57123

D835 DF23

55349 57149

D835 DF3D

55349 57133

D835 DF2D

55349 57169

D835 DF51

Numeric character reference

𝜣

𝜣

𝜽

𝜽

𝜭

𝜭

𝝑

𝝑

Character

흝

흷

횹

힋

Unicode name

MATHEMATICAL SANSSERIF
BOLD CAPITAL THETA

MATHEMATICAL SANSSERIF
BOLD SMALL THETA

MATHEMATICAL SANSSERIF
BOLD CAPITAL THETA SYMBOL

MATHEMATICAL SANSSERIF
BOLD THETA SYMBOL

Encodings

decimal

hex

decimal

hex

decimal

hex

decimal

hex

Unicode

120669

U+1D75D

120695

U+1D777

120505

U+1D6B9

120715

U+1D78B

UTF8

240 157 157 157

F0 9D 9D 9D

240 157 157 183

F0 9D 9D B7

240 157 154 185

F0 9D 9A B9

240 157 158 139

F0 9D 9E 8B

UTF16

55349 57181

D835 DF5D

55349 57207

D835 DF77

55349 57017

D835 DEB9

55349 57227

D835 DF8B

Numeric character reference

𝝝

𝝝

𝝷

𝝷

𝚹

𝚹

𝞋

𝞋

Character

힗

ힱ

힠

ퟅ

Unicode name

MATHEMATICAL SANSSERIF
BOLD ITALIC CAPITAL THETA

MATHEMATICAL SANSSERIF
BOLD ITALIC SMALL THETA

MATHEMATICAL SANSSERIF
BOLD ITALIC CAPITAL THETA SYMBOL

MATHEMATICAL SANSSERIF
BOLD ITALIC THETA SYMBOL

Encodings

decimal

hex

decimal

hex

decimal

hex

decimal

hex

Unicode

120727

U+1D797

120753

U+1D7B1

120736

U+1D7A0

120773

U+1D7C5

UTF8

240 157 158 151

F0 9D 9E 97

240 157 158 177

F0 9D 9E B1

240 157 158 160

F0 9D 9E A0

240 157 159 133

F0 9D 9F 85

UTF16

55349 57239

D835 DF97

55349 57265

D835 DFB1

55349 57248

D835 DFA0

55349 57285

D835 DFC5

Numeric character reference

𝞗

𝞗

𝞱

𝞱

𝞠

𝞠

𝟅

𝟅

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.
See also
Notes

^ http://math.stanford.edu/~feferman/papers/idsaga.pdf

^ ^{a} ^{b} Barry, Kieren (1999), The Greek Qabalah: Alphabetic Mysticism and Numerology in the Ancient World, .

^ Kaplan, Robert (2000), The Nothing that Is: A Natural History of Zero, Oxford; New York: .
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