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Page 195 of the TeXBook reads:

Although formulas within a paragraph always break after binary operations and relations, displayed formulas always break before binary operations and relations.

What is the reason for this rule?

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(Is this question acceptable here? It's not specifically about TeX, just about math typesetting.) – Ben Alpert Jun 19 '11 at 18:30

That statement by Knuth regarding displayed equations is Knuth's opinion. It is not a universally accepted typography rule. From a quick glance at multiple books and journals, there apparently is not a universally accepted typography rule.

  • Some use the rule as espoused by Knuth.
  • Some use the opposite rule: displayed equations are split after a binary operator.
  • You would think that those are the only choices. They aren't. Some take a third route: Repeat the binary operator at the end of one line and at the start of the next.
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Is there any reason for this rule, though? Just how Knuth felt on the day that he was writing the book? – Ben Alpert Jun 19 '11 at 19:39
@Ben: You'll have to ask Knuth. I can see compelling arguments for all three methods. – You Jun 19 '11 at 19:51
The rule espoused by Knuth isn't his. He was repeating conventional wisdom. This appears to be the most commonly used of the three, but all three are in use, particularly the first (break before a binary operator) and the last (repeat the broken operator). – David Hammen Jun 19 '11 at 20:40
Some typographical rules are, in fact, a mere convention. However, some of those conventions have been collected in books that serve as guidelines. In the updated edition of 1999 of the classic Mathematics into Type by Ellen Swanson, the rule is mentioned in Section 3.3.4 Rules for breaking equations in display: "In display, equations may be broken before an operator but not after an operator". (I don't own the 1971 first edition, but surely this same rule was there?). – Gonzalo Medina Jun 19 '11 at 22:37
When I write code, I break long mathematical expressions after a binary operator. That dangling operator at the end of the line gives a clue to the reader (or maintainer) that there is more to the expression at hand. When I document that code, I break before the binary operator. Think of sum = (long expression) - (second long expression) + (third long expression) - (final long expression). Because of how subtraction works, pairing the operator with the subtrahend/addend as opposed to hiding the operator at the end of the previous line improves comprehensibility (at least for the sighted). – David Hammen Jun 19 '11 at 23:05

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