The High German words machen (make), brechen (break), Becher (beaker), Buch (book), Lerche (lark), Milch (milk), Koch (cook) and Woche (week) all resulted from a sound shift occurring between the 4th and 5th century AD. The plosive ‘k’ sound shifted to a fricative ‘ch’ (represented in the International Phonetic Alphabet by /xx/ or /x/) between vowels and at the ends of words. Attempting to produce a ‘k’ sound after back vowels (a, o, u) while keeping the tongue low in the mouth will produce a sound that is sometimes described as harsh. After frontal vowels such as e or i it will sound more like the ‘h’ in the word ‘human’ does for many English speakers.
Interestingly, High German often retained the ‘k’ sound when English evolved it into ‘ch’, a process known as k-fronting. Examples of this divergence are found in words such as finch (Fink), birch (Birke) and chin (Kinn).