Left: hill 阝, right: examine carefully (originally two people 兄 discussing under a roof. Better in respect to 'examine' 験: a person 人 examining the horse's 馬 mouth 口. [ are the mane and eyes 一 ])
Mnemonic
(To avoid rockfalls:) The hill has to be examined carefully because it is steep and precipitous.
The radical kozato shows a hill. 阝 and is always on the left side of a kanji. (Please note that rather similar radical oozato⻏ is always on the right sight of the kanji and means village.)
Due to the use in “to examine 験”, this radical is seen as the head of a horse (with mane and eyes 一), in front of which a person stands 人 and checks/examines the mouth 口.
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