'TH' is a digraph that we mark just as a blend, however digraphs differ from blends in that they are two consonants standing together to produce one sound. A voiceless consonant produces no vibration in the vocal cords.
The voiced sound of 'th' is found in words such as then, there, this and that. The voiceless sound of 'th' is found in words like thick, thin, with, math, thud and think.
The vowel 'e' is a very strong and controlling vowel. When 'e' follows 'th' it changes it from voiceless to voiced. When adding 'e', bath changes to bathe, breath changes to breathe and cloth changes to clothe.
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