An orthography based on the principle that symbols correspond to phonemes may, in some cases, lack characters to represent all the phonemes or all the phonemic distinctions in the language. This is called a defective orthography. An example in English is the lack of any indication of stress. Another is the digraph "th", which represents two different phonemes (as in "then" and "thin") and replaced the old letters "ð" and "þ". A more systematic example is that of abjads like the Arabic and Hebrew alphabets, in which the short vowels are normally left unwritten and must be inferred by the reader.