In Korean, there are five fortis (된소리) 'ㄲ', 'ㄸ', 'ㅃ', 'ㅆ',
'ㅉ'

>**된소리 법칙(law) :** Sound between two vowels
>
>**Example :** 소쩍새 (scops owl) -> 솢쩍쌔, 어깨 (shoulder)-> 억깨,  
오빠 (brother)->옵빠, 으뜸 (best) -> 읃뜸

**Irregular :** 깨끗하다 (clean) -> 깨끗타다

Here, if we follow law, then 깩끗타다. But by ㄲ of 깨, we have
깨끗타다.