The Relationship Between "M" and "B"

To native English speakers, it’s not apparent that m and b have much in common. After all, the English words "man" and "ban" sound very different. However, to native Japanese speakers, those first consonants sound fairly close. As it happens, there is a lot of m/b interchangeability in Japanese words.

Here's one example:

寂しい (さびしい or さみしい: (1) lonely; (2) solitary; (3) desolate)

Both readings are perfectly valid, but the first is a Joyo kun-yomi for 寂 (along with the similar-sounding さび and さび•れる), whereas さみしい is non-Joyo. 

It's also fine to read the next word with an m or a b:

瞑る (つむる or つぶる: to shut (eyes))

The kanji is non-Joyo.

Here's a third example:

こむら返り or 腓返り (こむらがえり or こぶらがえり: leg cramp) 

The non-Joyo 腓 means "calf (in leg)."

Which Reading Has Prevailed?

We can read the following terms with interchangeable consonants, but in each case the second yomi sounds old-fashioned:

煙い (けむい or けぶい: smoky)

煙る (けむる or けぶる: (1) to billow smoke; smolder; (2) be hazy; look dim)

In the following examples, m has again prevailed, whereas the b reading has completely fallen into disuse:

守る	to protect	まもる	まぼる
紐	string	ひも	ひぼ
侍	samurai	さむらい	さぶらい
眠る	to sleep	ねむる	ねぶる
弔う	to mourn	とむらう	とぶらう
葬る	to bury (the dead)	ほうむる	はぶる
睦ぶ	to be harmonious	むつむ	むつぶ

Some notes:

• I have reddened the older of the two readings. 

• The 紐 kanji is non-Joyo.

• In most cases, the b version is the original yomi. And it makes sense that that earlier reading has become obsolete, as ancient things tend to cede to newfangled ones. With the top two lines, though, the older version "won."

• As you can see, the yomi of 葬る changed in another way, besides the b-to-m evolution, in that は turned into ほう.

• For 睦ぶ, the むつぶ is probably older because it (not むつむ) appeared in The Tale of Genji, an 11th-century work. 

The b reading is current in the next terms, whereas the m version is obsolete:

暫く	for awhile	しまらく	しばらく
乏しい	deficient	ともしい	とぼしい
罅 (non-Joyo)	crack	ひみ	ひび

I have again reddened the original readings. 

Here's another example in which b has more or less prevailed:

貴ぶ or 尊ぶ (とうとぶ or たっとぶ or とうとむ or たっとむ: to value, prize; esteem, respect)

The first and third yomi go together from a b/m perspective, just as the second reading corresponds to the fourth one.

People seldom use the m readings of this verb nowadays. Of the two b readings, とうとぶ is older. 

It’s hard to find a deeper reason for any of these m and b changes. Overarching patterns also prove elusive.

The M/B Relationship in On-Yomi

We saw that when it comes to the kun-yomi of 睦, むつぶ has yielded to むつむ. Now let's check out the on-yomi of that kanji:

呉音 (ごおん, from Chinese readings of the 5th–6th c.): モク [unused now]

漢音 (かんおん, from Chinese readings of the 7th–8th c.): ボク

The older version (モク) contains an m and was discarded in favor of the b yomi (ボク). In other words, the kun readings evolved from b to m, whereas the two on-yomi went in the other direction! 

Like 睦, quite a few kanji have on-yomi with m/b relationships. Here are other examples, with any Joyo yomi colored purple:

Kanji	Definition	呉音	漢音
漠 (1700)	desert	マク	バク
膜 (1834)	membrane	マク, モ	バク, ボ
寞 (non-Joyo)	lonely	マク	バク
幕 (977)	curtain	マク	バク
望 (585)	to hope	モウ	ボウ
模 (980)	model	モ	ボ
苗 (1740)	seedling (esp. rice)	ミョウ	ビョウ
描 (1741)	to depict	ミョウ	ビョウ
猫 (1742)	cat	ミョウ	ビョウ, ボウ

Some of the black yomi are unused now.

If you've noticed shared shapes in kanji such as 漠 and 膜 on the one hand or 苗 and 描 on the other, indeed there are on-echo patterns at work here. (For an explanation of that term, which I coined, see the third-to-last section of the Glossary.)

An Outlier

Here's a weird situation:

木守り (きまもり or きまぶり: fruits left on a tree during winter (supposedly causing more fruits to appear in the next season))

This very cool term is unfortunately archaic. Anyway, here we see きまもり versus きまぶり. That is, も corresponds to ぶ, not to ぼ. 

I mentioned this oddity to Mark Spahn (who wrote The Kanji Dictionary along with Wolfgang Hadamitzky) because Spahn is the one who initially brought the m/b relationship to my attention over several emails. He replied that maybe まぶり is an archaic version of まもり. 

I checked this idea out with my proofreader, who said he has no definitive answer. He added, "Dictionaries have まぶり and まぼり (which appeared above) as synonyms of まもり," but there's no indication of which is the original word. 

Having no definitive answers about these evolutionary processes seems to be the general state of affairs. And when a b or m yomi persists beyond its time of usefulness, isn't it a bit like a fruit left on a tree during winter?!