[Tex/LaTex] Breaking formulas before and after binary operators

Question

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?

Answer 1

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.