The IPA symbol for the “th” sound in English “thin” is θ.
This sound is called the voiceless dental fricative. To produce it, the tongue is placed against or between the teeth while air is forced through the narrow opening, creating friction. The vocal folds do not vibrate, which makes the sound voiceless. In ordinary spelling, English represents it with the letters th, as in “thin,” “think,” and “bath.”
English has another “th” sound, represented in the IPA by ð, the voiced dental fricative. It occurs in words such as “this,” “that,” and “breathe.” The distinction is based on voicing, not spelling: both sounds use the same th digraph, so learners must identify the pronunciation.
The symbol θ is the lowercase Greek letter theta. It should not be confused with ʃ, the “sh” sound, or ŋ, the sound at the end of “sing.” In phonetic transcription, θ precisely records the consonant rather than the two written letters.