73. The "Flat Sun" Radical: 曰

These two shapes may look identical, but a closer examination reveals otherwise:

日 (62: sun, day, date; Sunday; Japan)

曰 (to say), a non-Joyo kanji

The latter character is broader, and the cross stroke actually only goes three-fourths of the way across 曰. 

These shapes are also separate radicals, at least theoretically:

日 (radical 72: "sun")

曰 (radical 73: "flat sun"), referred to in Japanese as ひらび (meaning 平日, "flat sun") or いわく (the kun-yomi of 曰く

Kanjigen does indeed list them as different radicals, but Nelson treats radical 73 as a variant of radical 72.

Here's my thinking. Radical 73 is on duty in these Joyo kanji, according to Kanjigen:

書 (142: writing; book)
曲 (261: piece of music, song; curve)
最 (484: most)
(1248: furthermore; to renew; change; grow late; brand-new; night watch)
曹 (1520: low-ranking government official; sodium)
替 (1548: exchange; replacement)
(2052: previously; great- (in family lineage))

That is, the 日 or 日-like parts used to be 曰. But those shapes now look exactly like 日, so I feel it only makes sense to classify all seven characters under radical 72.

To find out about that radical, see Radical Note 72.

Photo Credit: Eve Kushner

A calligrapher drew this 書 in such a way as to make the 日 unrecognizable.