What characteristic makes English a "deep" alphabetic orthography?

The characteristic that makes English a "deep" alphabetic orthography is that its spelling system represents meaningful parts (morphemes) as well as sounds. In a deep orthography, letters not only correspond to phonemes (the individual sounds) but also reflect the structure of the language at the level of meaning. This means that English spelling can illustrate morphological relationships, enabling readers to understand connections between words that share the same root or affix, even if their pronunciations vary.

For example, the words "happy," "happiness," and "unhappy" share the morpheme "happy," illustrating how knowledge of a word's components can aid comprehension. This aspect of English orthography contributes to its complexity, as words can be phonetically similar yet vary in meaning and usage. The presence of morphemes means that understanding the underlying structure of words is essential for proficient reading and spelling in English.