3. The "Dot" Radical: 丶

The "dot" radical 丶 has worked its way into several kanji without contributing much of a meaning. Thus, associating characters with this radical is often just a random act of organization; each kanji needs a home, and radical 3 might as well be it.

Which Kanji Contain the "Dot" Radical?

Only four Joyo kanji feature 丶 as an on-duty radical:

主 (299: to preside over; deity; main; you)

丸 (830: round; complete; Japanese period; good, correct)

丹 (1563: vermilion; sincerity; mental energy)

丼 (2083: bowl; topped rice bowl)

Incidentally, the dots in three of those kanji distinguish them from undotted characters:

王 (5: king, magnate), where 王 (radical 96: "jewel") is the on-duty radical

九 (12: nine), where the on-duty radical is 乚, a variant of 乙 (radical 5: "fishhook")

井 (1470: well; town), where 二 (radical 7: "two"), is on duty

Meanwhile, 丹 contrasts with this more-dotted kanji: 

舟 (1354: small boat), where 舟 (radical 137: "boat") is on duty

I mention the radicals of those differently dotted kanji to show how the presence of our 丶 changes the radical categorization.

What Does the "Dot" Radical Mean?

The radical in 丸 and 丼 is so semantically insignificant that Henshall says nothing about those dots in the etymologies from his newer edition.

For 主, he says that this character originally meant "lamp" and that an early form depicts a "lamp with a wick burning." I guess the dot represents the top of the wick.

Only in 丹 does our radical contribute meaning. The whole character depicts a "pit," with the middle mark likely representing an "excavated red substance, specifically vermilion ore," according to Henshall.

What Do the Japanese Call the "Dot" Radical?

The Japanese primarily refer to the 丶 radical as てん. To my great surprise, 丶 is a non-Joyo kanji that means "mark, dot" and that carries the yomi チュ.

The one-stroke 丶 radical resembles the Japanese comma, also called てん. 

An alternate Japanese name for the radical is ちょぼ. That word comes from 樗蒲 (ちょぼ: gambling, where both kanji are non-Joyo), an allusion to the pips on dice. (The same term plays a role, albeit in hiragana form, in おちょぼ口 (おちょぼぐち: button of a mouth), suggesting that the small, pretty lips of a woman or child resemble a dot.)

Incidentally, Denshi Jisho provides "tick" radical as an alternate radical name in English, but that makes me think of Lyme disease, so I'm happy to go with "dot."

Photo Credit: Eve Kushner

This enormous ship moored in Tahiti is called 天宝丸 (てんほうまる). The 天宝 part likely means "heavenly treasure," and 丸 is a suffix for ship names.

We're up to speed on 丸, so let's turn our attention to 宝 (971: treasure). One might mistakenly perceive a dot on top, but that's actually part of the on-duty radical 宀 (40: "katakana u"). As for the dot on the lower right, that belongs to 玉, the off-duty radical 102 ("jewel"). 

In his etymology of the autonomous kanji 玉 (15: ball; spherical object; jewel), Henshall says that 玉 and 王 (king) were indistinguishable at one point. Then 玉 came to be written with a dot, "no doubt to help distinguish it from 王." In the game shogi, by the way, one player's king is called 王将 (おうしょう), whereas the opponent's king is 玉将 (ぎょくしょう)!

Don't See the 丶 Radical Everywhere

As the discussion of 宝 demonstrates, it's easy to start seeing dots everywhere in kanji. It's also usually misleading; those little marks tend to belong to more elaborate radicals. A general tip with identifying radicals is to look for the bigger entity. When it comes to 丶, the dot might be part of radicals such as these:

Photo Credit: Eve Kushner

In this sign from Osaka, we again spot 丸 and 宝, quite coincidentally. The third black character has an extremely odd form. It's usually written as 鮮, but the left-side 魚 has been replaced with its variant. As for the right-side 羊, the dots on top have been rendered in one fell swoop, as if they were a single stroke, which often happens in handwriting. The proper form 鮮 and the subsequent 魚 contain lots of dots, which are actually part of radical 195, 魚 ("fish"). Finally, the little mark atop 店 belongs to radical 53, 广 ("dotted cliff"). 

The five characters represent the name of a seafood shop: 

丸宝 (まるほう: lit. "round treasure"), where I've guessed at the yomi

鮮魚店 (せんぎょてん: fish dealer)

As for the odd shape in red, 㐧, that's the ryakuji (略字: simplified form) of 第. The 第2 (だいに) probably means "second," as in "second floor."

Photo Credit: Eve Kushner

In this photo of a vendor's booth in Taiwan, 丸 appears once again! It's in both the circle before 日本丸 (にほんまる) and at the end of that word. I imagine that 日本丸 is the name of a real or fictitious fishing ship.

With the profusion of 魚s, we see lots of dots that, as we know, are part of radical 195, 魚 ("fish").

Two of the menu choices end in 丼. Chinese speakers once used this character, which was synonymous with 井 (well). The dot in the center of 丼 represented the water in the well. They then dropped 丼 from their "library" of characters. Subsequently, the Japanese invented 丼, making it represent "rice bowl topped with other items." The 井 depicted the container, and the dot symbolized the ingredients on the rice. After that, the Chinese borrowed 丼 with its rice-related meaning from Japanese.