コメント欄にもブックマークにもこの仮説が出ていなかったので。

On Off and Beyond: ○×△の起源は？
しかし、よく考えると、○　×　もさることながら、なぜ△がその間のレベルなんでしょうか。

私の仮説は、「あれはもともと半円だったのではないか」ということ。しかし、半円をきれいに描くのは以外と難しい。それが「なまった」結果、△になったのではないか。

それよりも、×が「ばってん」になった理由がわからない。こちらは○と違って欧米でも no の意味で使うのだ。

ちなみに△に相当する点数付けは、日本の方が合州国よりむしろ多かったように思う。close but wrong の場合はやはり×という場合が多く、どうしても点数を上げたい場合は、1/2とか-n (n = 減点数)とかというexplicitな書き方をしていた。

X mark - Wikipedia, the free encyclopedia
An x mark (also known as a cross, x, ex, or exmark) is a mark (x, ×, X, ?, ?, ?, ?, ?, etc.) used to indicate the concept of negation (for example "no, this has not been verified" or "no, I don't agree") as well as affirmation (for example in election ballot papers or in x marks the spot). It is often used opposite the checkmark (or the O mark used in Japan).

なお、○がyesの意味であることは、今では英語圏でも知られている模様である。ゲーム機のコントローラーの功績だろうか。

Tick (checkmark) - Wikipedia, the free encyclopedia
In Japan and Korea, an "O Mark" (in the appearance of a circle, unicode symbol "◯"), also known as "丸印" marujirushi, is used instead of a tick to mean 'yes'.

もっとも、ある記号を否定するのに、右から左に線を引くのはかなり一般的で、Unicodeを"NOT"でgrepしただけでこんだけ出てきた。

文字コード文字名
¬U+00ACNOT SIGN
͊U+034ACOMBINING NOT TILDE ABOVE
∄U+2204THERE DOES NOT EXIST
∉U+2209NOT AN ELEMENT OF
∌U+220CDOES NOT CONTAIN AS MEMBER
∤U+2224DOES NOT DIVIDE
∦U+2226NOT PARALLEL TO
≁U+2241NOT TILDE
≄U+2244NOT ASYMPTOTICALLY EQUAL TO
≆U+2246APPROXIMATELY BUT NOT ACTUALLY EQUAL TO
≉U+2249NOT ALMOST EQUAL TO
≠U+2260NOT EQUAL TO
≢U+2262NOT IDENTICAL TO
≨U+2268LESS-THAN BUT NOT EQUAL TO
≩U+2269GREATER-THAN BUT NOT EQUAL TO
≭U+226DNOT EQUIVALENT TO
≮U+226ENOT LESS-THAN
≯U+226FNOT GREATER-THAN
⊀U+2280DOES NOT PRECEDE
⊁U+2281DOES NOT SUCCEED
⊄U+2284NOT A SUBSET OF
⊅U+2285NOT A SUPERSET OF
⊊U+228ASUBSET OF WITH NOT EQUAL TO
⊋U+228BSUPERSET OF WITH NOT EQUAL TO
⊬U+22ACDOES NOT PROVE
⊭U+22ADNOT TRUE
⊮U+22AEDOES NOT FORCE
⋠U+22E0DOES NOT PRECEDE OR EQUAL
⋡U+22E1DOES NOT SUCCEED OR EQUAL
⋢U+22E2NOT SQUARE IMAGE OF OR EQUAL TO
⋣U+22E3NOT SQUARE ORIGINAL OF OR EQUAL TO
⋤U+22E4SQUARE IMAGE OF OR NOT EQUAL TO
⋥U+22E5SQUARE ORIGINAL OF OR NOT EQUAL TO
⋦U+22E6LESS-THAN BUT NOT EQUIVALENT TO
⋧U+22E7GREATER-THAN BUT NOT EQUIVALENT TO
⋨U+22E8PRECEDES BUT NOT EQUIVALENT TO
⋩U+22E9SUCCEEDS BUT NOT EQUIVALENT TO
⋪U+22EANOT NORMAL SUBGROUP OF
⋫U+22EBDOES NOT CONTAIN AS NORMAL SUBGROUP
⋬U+22ECNOT NORMAL SUBGROUP OF OR EQUAL TO
⋭U+22EDDOES NOT CONTAIN AS NORMAL SUBGROUP OR EQUAL
⌐U+2310REVERSED NOT SIGN
⌙U+2319TURNED NOT SIGN
⍯U+236FAPL FUNCTIONAL SYMBOL QUAD NOT EQUAL
⍻U+237BNOT CHECK MARK
⨔U+2A14LINE INTEGRATION NOT INCLUDING THE POLE
⪇U+2A87LESS-THAN AND SINGLE-LINE NOT EQUAL TO
⪈U+2A88GREATER-THAN AND SINGLE-LINE NOT EQUAL TO
⪉U+2A89LESS-THAN AND NOT APPROXIMATE
⪊U+2A8AGREATER-THAN AND NOT APPROXIMATE
⪱U+2AB1PRECEDES ABOVE SINGLE-LINE NOT EQUAL TO
⪲U+2AB2SUCCEEDS ABOVE SINGLE-LINE NOT EQUAL TO
⪵U+2AB5PRECEDES ABOVE NOT EQUAL TO
⪶U+2AB6SUCCEEDS ABOVE NOT EQUAL TO
⪹U+2AB9PRECEDES ABOVE NOT ALMOST EQUAL TO
⪺U+2ABASUCCEEDS ABOVE NOT ALMOST EQUAL TO
⫋U+2ACBSUBSET OF ABOVE NOT EQUAL TO
⫌U+2ACCSUPERSET OF ABOVE NOT EQUAL TO
⫬U+2AECDOUBLE STROKE NOT SIGN
⫭U+2AEDREVERSED DOUBLE STROKE NOT SIGN
⫮U+2AEEDOES NOT DIVIDE WITH REVERSED NEGATION SLASH
⽆U+2F46KANGXI RADICAL NOT
⽏U+2F4FKANGXI RADICAL DO NOT
ꆊU+A18AYI SYLLABLE NOT
￢U+FFE2FULLWIDTH NOT SIGN

だとしたら、×(ばってん)単にstrike outを強調したということなのかも知れない。

Dan the Pictographer

P.S. そういえば Unicode にははなまるがない。ケータイ絵文字にはあったっけ?