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The PreTeXt Guide

Subsection 4.9.3 Multi-line Display Mathematics

We begin with a pure container, either <md> or <mdn>. The former numbers no lines, the latter numbers every line. Within the container, content, on a per-line basis, goes into a <mrow> element. You can think of <mrow> as being very similar to <me>. In a align the \\ mark the end of a displayed line. In PreTeXt each <mrow> delimits a displayed line, and there are no \\s. Use \amp to mark the alignment point.
On any given <mrow> you can place the @number attribute, with allowable values of yes and no. These will typically be used to override the behavior inherited by the container, but there is no harm if they are redundant. A given line of the display may be the target of a cross-reference, though the numbering flexibility means you can try (and fail) to target an unnumbered equation.
An <mrow> may have a @tag attribute in place of a @number attribute. This will create a “number” on the equation which is just a symbol. This is meant for situations where you do not want to use numbers, and the resulting cross-reference is “local.” In other words, the <xref> and its target are not far apart, such as maybe within the same <example> or the same <proof>. Allowable values for the attribute are:
star, dstar, tstar,
dagger, ddagger, tdagger,
daggerdbl, ddaggerdbl, tdaggerdbl,
hash, dhash, thash,
maltese, dmaltese, tmaltese
These are the names of symbols, with prefixes where the prefix d means “double”, and the prefix t means “triple”. Cross-references to these tagged equations happens in the usual way and should behave as expected. See Section 3.4 and Section 4.5 for more on cross-references.