### 3.2.4 Reference to page numbers

A particular place of a score can be marked using the `\label` command, either at top-level or inside music. This label can then be referred to in a markup, to get the number of the page where the marked point is placed, using the `\page-ref` markup command.

    \header { tagline = ##f }
    \book {
      \label #'firstScore
      \score {
        {
          c'1
          \pageBreak \mark A \label #'markA
          c'1
        }
      }
      \markup { The first score begins on page \page-ref #'firstScore "0" "?" }
      \markup { Mark A is on page \page-ref #'markA "0" "?" }
    }

The `\page-ref` markup command takes three arguments:

1. the label, a scheme symbol, eg. `#'firstScore`;
2. a markup that will be used as a gauge to estimate the dimensions of the markup;
3. a markup that will be used in place of the page number if the label is not known;

The reason why a gauge is needed is that, at the time markups are interpreted, the page breaking has not yet occurred, so the page numbers are not yet known. To work around this issue, the actual markup interpretation is delayed to a later time; however, the dimensions of the markup have to be known before, so a gauge is used to decide these dimensions. If the book has between 10 and 99 pages, it may be "00", ie. a two digit number.

#### Predefined commands

`\label`, `\page-ref`.

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