21. The "Katakana Hi" Radical: 匕
The "katakana hi" radical 匕 is on duty in only two Joyo kanji (and perhaps just one, as we'll see):
北 (205: north; to flee)
化 (238: to change; disguise oneself; bewitch)
The 匕 shape also serves as a component in many other characters, including 花 (flower) and 指 (finger).
When a radical is as simple as the two-stroke 匕, the meaning of the shape tends to be less clear than that of pictorial radicals. And so it is with 匕.
On 北
Henshall calls 北 a "pictograph of two people back-to-back in profile" in his newer edition (the source of all his etymologies in this Radical Note). Thus, 北 originally meant "to turn back on" or "retreat." He considers "north" a loan usage or perhaps an extended sense; the idea is that ancient Chinese houses faced south, so occupants turned their backs to the north.
Photo Credit: Eve Kushner
The Names of the 匕 Radical
One might search for the meaning of 匕 in its nomenclature. Known as "katakana hi" or "spoon" in English, the radical goes by these names in Japanese:
さじのひ
さじ
ひ
Apparently, the さじ comes from the yomi of the non-Joyo 匙 (spoon). Because 匙 contains that on-duty radical, someone probably thought to associate 匕 with 匙 in these radical names. Actually, our radical doubles as a non-Joyo kanji that means "spoon" (and "knife"!) and that has the same さじ pronunciation. But the Japanese connect spoons much more with 匙 than with 匕, which they almost never use as an autonomous character. For this reason, my proofreader suspects that the reading of 匙 gave rise to the yomi of our radical.
And then ひ is the hiragana equivalent of the katakana ヒ. (Note the minuscule differences in that kana versus radical 21. Most notably, the lower right part of our 匕 radical curves upward, whereas the kana ヒ sticks to the baseline.)
Incidentally, don't call the 匕 radical ひ because that's unclear, as two other radicals—日 (radical 72: "sun") and 火 (radical 86: "fire")—share that name.
Primarily, the Japanese refer to 匕 as さじのひ, combining the ideas of the spoon and katakana ヒ.
The Meanings of 匕
I discovered something about the meaning of 匕 in Henshall's etymology for this kanji:
化 (238: to change; disguise oneself; bewitch)
He says that the right side represents "person fallen down," which indicates a "change of state," and he notes that the 亻 means "person standing."
Until I saw his analysis, I didn't realize that sources disagree about the radical of 化. Kanjigen goes with 匕, as does Joy o' Kanji. By contrast, Henshall and Denshi Jisho classify the character under 亻, a variant of 人 (radical 9: "person"). Siding with them would leave 北 (205: north; to flee) as the only Joyo kanji featuring an on-duty 匕 radical.
The Variant of 匕
In his etymology of 化, Henshall illuminates another issue. Whereas the Japanese render the 匕 radical as we're seeing it here, the Chinese do it as 七. That shape, which Nelson lists as a variant form of our radical, appears in the following photo.
Photo Credit: Yoshikazu Kunugi
In the photo above, the radical of 化 looks a lot like 七, the kanji for "seven," which made me wonder about the radical of that character. It turns out not to be 匕 but rather 一 (radical 1: "one")! Go figure! Henshall says in his etymology of the 七 character that it derives from a pictograph of a "cut bone."
Kanjigen similarly notes that 七 originally meant “to cut,” as in cutting the vertical line with the horizontal one. The same source also says that 七 once constituted the whole character now written as 切 (156: to cut). The 七 shape appears in just two Joyo kanji, the other being 叱 (2026: to scold). In the last section of "Oddball Additions to the Joyo Set" I discuss the quirkiness of the 叱 shape.
It seems that very little is straightforward when it comes to our radical! Both the meaning and the form are rather slippery!