A trigraph is a group of three letters together that make up a single phoneme, or sound, within a spoken language. What differentiates this type of group is that the phoneme they make is not the sound that they would make if all three were simply pronounced in order. Pronunciation of a trigraph creates a new sound that is unique to that particular arrangement of letters, such as the sound of “sch” in a word like “schwa.” These are rather uncommon in English, while digraphs that consist of only two letters together as a phoneme are quite common.
The primary components of a trigraph are that it consists of three letters and creates a vocal sound that is unusual or unique for its particular grouping. While a grouping like the suffix “-ing” does consist of three letters together, the sound that is created by them is simply each letter pronounced in order. Similarly, an arrangement of letters in a word like “tea” may seem like a trigraph, but it is not. The “t” at the beginning of the word is simply pronounced normally, and followed by a digraph that consists of the two vowels “e” and “a” to create a unique phoneme.
“Sch” can be a trigraph in some words, though there are also words in which it is not. In English words like “schilling” or the British pronunciation of “schedule,” these three letters come together to create a phoneme with a sound more commonly spelled as “sh.” In contrast to this, however, is a word like “school” or the American pronunciation of “schedule” in which the sound of the three letters is “sk” and the “h” becomes silent. The use of “sch” in these words is neither a trigraph nor a digraph, since they merely take on the expected pronunciation of “s” followed by “c,” rather than a new sound.
There are also some vowel combinations that can create trigraphs, though these are fairly unusual in English words that are not directly borrowed from another language. The word beau in French, for example, includes a trigraph consisting of the three vowels “eau” that create a phoneme similar to a long “o.” This is a common phoneme in French words, and many words from French that are used in English feature this sound. Borrowing and exchanging of such sounds can become confusing for some developing speakers, however, since the English word “beautiful” contains the same trigraph, but the sound they create is quite different than in “beau.”