The “th” sound is one of the most distinctive of the English language. For many foreigners it is also one of the most difficult- “fire ze cigarettes missiles!” declared the Frenchman in the viral video “End of Ze World?” The reason that pronouncing the definite article is ornery for most French speakers is that the “th” sound simply does not exist in the French language. Although the “z”, “d”, or “f” sounds used by non-English speakers from across the globe may be close approximations of “th”, “zese”, “dose”, and “fis” will always sound slightly foreign to the English ear.
Like all sounds, the production of “th” can be reduced to a physical process. It is made by sticking the tongue slightly beyond the upper front teeth and pushing air through the space. When we activate our vocal cords during production of this sound we call it “voiced”; if they remain inactive it is “unvoiced.” If you want to check if you are making a voiced or an unvoiced sound simply place your hand on your throat- vibration indicates vocal cord activity.
Although they are designated the same way in written language, voiced and unvoiced “th” are distinctive sounds; the International Phonetic Alphabet (IPA) refers to them as ð and θ, respectively. The second symbol will be familiar to many people from high-school trigonometry courses; although it does not represent an angle here but rather the Greek letter “theta.” In fact, the Greek language possesses the unvoiced “th” sound and uses it in words such as θεωρία (theory).
Those who have completed more advanced mathematical studies may have seen the other “th” symbol as well; ð is sometimes used in partial derivatives, although this operation is more commonly represented by the lower-case Greek “delta” δ. In fact while in Ancient Greek δ represented a “d” sound, in the modern version of the language it is pronounced as the voiced “th” ð sound– such as in Δαίδαλος (Daedalus).
However unlike θ, ð comes to us not from the balmy blue waters of the Aegean but from the icy North Sea. In the Icelandic language the letter is known as “Eð” and is used in words such as bróðir (brother). In Faroese the symbol appears for more etymological reasons and actually indicates a glide between two vowels- in the expression góðan morgun (good morning) the ð simply indicates a transition sound between the “o” and the “a”, in this case with the semi-vowel “w.” ð can therefore be considered a type of “false friend” in the Faroese language, which does not have the voiced “th” sound. In fact, most of the Northern Germanic languages no longer have the ð (aside from Icelandic, the other exception is the Elfdalian dialect of Swedish); the symbol was eventually replaced by the letter “d” and the “th” sound was lost, except for in Danish.
The ð was a letter in Old English as well. The letter which the Anglo-Saxons referred to as ðæt survived in written Old English until around 1300. Another symbol- þ, known as “thorn”- was used interchangeably with ð in Old English texts and survived somewhat longer. It eventually morphed shape to look something like a Y, leading to signs such as “Ye Olde”, which really should be pronounced “The Old.”
The adoption of printing presses limited to Latin characters was probably a major reason behind the disappearance of þ from the English alphabet (although it continues to be used in Icelandic to represent θ), but the sounds which it denoted have continued to be an important part of the language. They are also fairly unique; few major languages use the θ and ð sounds. The Arabic language is an exception; it uses both ð and θ- written as ﺫ and ﺙ, respectively.
Additionally, although the “th” sounds do not officially exist in Spanish or Portuguese, many speakers use them anyway in words such as “Sevilla”- a habit that is often mistaken for a lisp. Although the Spanish “lisp” is sometimes the butt of jokes, it certainly comes in handy in making a command such as “fire the missiles!” No matter how it is written- θ, ð, þ, Y, ﺙ, ﺫ, or s- pronouncing the “th” sound correctly is an imperative part of learning to properly speak English.