It's from the Japanese saying, 読書百遍意自ずから通ず(Repeated reading makes the meaning
clear). In this case, the ず ending doesn't make the verb negative. What is the
function and nuance of this usage? Is this a verb form from the old days
reserved for use in Japanese proverbs and perhaps some other specific and
narrowly defined instances? Another example where this is used in the same way
is 窮すれば通ず [きゅうすればつうず] (Necessity is the mother of invention). Is there another
example with another verb that uses the ず ending in the same way?
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conjugations
What's the meaning of ず in 通ず? (Not a negative)
I'm guessing it derives from `通{つう}ずる` (literary form `通{つう}ず` [according to
Daijisen](http://dic.yahoo.co.jp/dsearch?enc=UTF-8&p=%E9%80%9A%E3%81%9A%E3%82%8B&dtype=0&dname=0na&stype=0&pagenum=1&index=12293000)),
which is classified by edict as a "zuru verb alternative form of -jiru verbs"
(an alternative form of `通{つう}じる`).
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通ず comes from the verb 通ずる _(to lead to)_. In this case, ず is used as another
way to conjugate the 〜する/〜ずる verb.

References:

  * **[About henkaku doushi/irregular verbs](http://www.imabijapaneselearningcenter.com/lesson16.htm)** ( _Near the bottom of this page, the writer suggests that ずる is related more to classical Japanese, as you were suspecting._ )
  * ( **[サ行変格活用動詞](http://ja.wikipedia.org/wiki/%E3%82%B5%E8%A1%8C%E5%A4%89%E6%A0%BC%E6%B4%BB%E7%94%A8)** ・サぎょうへんかくかつようどうし _general reference_ )

**In answer to your question:**

> Noun + ずる verbs can also be written as Noun + ず.

Here are just a few examples [based off of this
post](http://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q1464126419) by
urutorasenpaiさん where this ず can commonly be seen:

  * 案{あん}ずる -> 案{あん}ず 
  * 感{かん}ずる -> 感{かん}ず
  * 弾{だん}ずる -> 弾{だん}ず

Seeing that 〜ず, in this case, seems to happen most often in ことわざ or classical
Japanese would seem to indicate that this is possibly more of a poetic,
stylistic conjugation and use of the verb. It helps with fitting verbs into a
smaller amount of syllables, as well.
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