23. The "Hiding" Radical: 匸

It is appropriate that 匸, radical 23, is called the "hiding" radical (or かくしがまえ in Japanese) because it presents us with a now-you-see-it-now-you-don't kind of situation.

In the past, these characters contained the 匸 radical:

医 (225: doctor; medicine)
区 (465: district; ward; area; to divide)

However, all instances of that radical have been replaced with 匚 (radical 22: "box on side") in Joyo kanji. One still sees traces of 匸 in dictionaries; notably, Kanjigen files those two kanji under 匸 but treats it as being the same radical as 匚. 

Joy o' Kanji also classifies 医 and 区 (the only two Joyo characters ever to have contained radical 23) under radical 22.

To find out about 匚, see Radical Note 22.