To reference a whole equation group and its individual equations, load amsmath, wrap the numbered display in subequations, and place a group label immediately inside that wrapper. Give each numbered row a separate label, then cite either level with \eqref. amsmath User’s Guide, §3.11.

This guide is for someone editing a LaTeX document on a Mac who needs references such as “system (1)” and “equation (1b).” The examples use abstract algebraic expressions and distinguish environments that number separate equations from those that only arrange multiple lines.

Create a group reference and individual equation references

Start with a working LaTeX installation. The LaTeX Project identifies MacTeX as a Mac distribution containing LaTeX, a complete TeX system, and document editors. The numbering technique below comes from the amsmath package. Getting LaTeX; CTAN: amsmath.

Save the following complete document as equation-group.tex in your LaTeX editor. The important placement is \label{eq:system} directly after \begin{subequations}, before the inner align begins. The package guide explicitly documents this position for capturing the parent number. amsmath User’s Guide, §3.11.3.

\documentclass{article}
\usepackage{amsmath}

\begin{document}

Consider the system~\eqref{eq:system}:
\begin{subequations}\label{eq:system}
\begin{align}
  a+b &= c \label{eq:sum} \\
  a-b &= d \label{eq:difference} \\
  2a  &= c+d \label{eq:combined}
\end{align}
\end{subequations}

The whole group is system~\eqref{eq:system}.
The second relation is equation~\eqref{eq:difference}.
Together, equations~\eqref{eq:sum} and~\eqref{eq:difference}
give equation~\eqref{eq:combined}.

\begin{equation}\label{eq:next}
  2b=c-d
\end{equation}

The next equation is~\eqref{eq:next}.

\end{document}

Compile the main file twice so that LaTeX can write and then read its reference information. From Terminal, when pdflatex is available and your working directory contains the saved file, run the following commands. LaTeX’s use of an auxiliary file explains why references can appear as ?? on the first run. LearnLaTeX: Cross-referencing.

pdflatex -halt-on-error equation-group.tex
pdflatex -halt-on-error equation-group.tex

For this standalone document, the expected references are listed below. These are predictions from the documented numbering behavior, not a report of a local compilation; no LaTeX compiler was available in the research environment.

Reference command Expected result What it identifies
\eqref{eq:system} (1) The complete group
\eqref{eq:sum} (1a) The first equation
\eqref{eq:difference} (1b) The second equation
\eqref{eq:combined} (1c) The third equation
\eqref{eq:next} (2) The equation after the group

The implementation explains the sequence: entering subequations advances and saves the parent equation number, then starts a subordinate counter. Leaving the wrapper restores the main counter to the parent value. Consequently, three children under parent (1) are followed by ordinary equation (2). The amsmath package, §15.

Choose the environment that matches your reference targets

Before choosing the layout, decide what a reader must be able to cite: the collection, each equation, or one expression broken across lines. This decision aid applies the distinctions in the package documentation. amsmath User’s Guide, §§3.2–3.7.

Reader’s need Supported structure Numbering target
Cite a group and each aligned equation subequations around align Parent plus numbered rows
Cite a group of separately centered equations subequations around gather Parent plus numbered equations
Cite related displays separated by prose One subequations around multiple numbered displays Parent plus each display
Cite one multiline expression equation containing split or aligned One equation number
Show aligned rows without automatic numbers align* No automatic row numbers

Why aligned does not create individual subequations

aligned is a building block inside an enclosing math environment. Using equation with aligned gives the enclosing equation a number; it does not establish a separate numbered target for each row. For individual row references, choose align inside subequations. The guide distinguishes these display environments from their subordinate building blocks. amsmath User’s Guide, §3.7.

A useful planning test is to draft the referring sentence first. If you need “the second equation in system (1),” the first example supplies both targets. If you only need “the derivation in equation (1),” a single numbered multiline display may be sufficient. Choose the structure before adding labels; this makes the intended reference targets explicit.

Keep one group across separate displays

The wrapper can include prose between its equations. The package’s distributed subequation examples demonstrate this, along with grouped displays using different mathematical environments. Close the wrapper after the last related display. Subnumbered equation examples.

Here is a second complete document. The first subequation occupies two printed lines but has one reference target; the second is a separate display after a sentence.

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{subequations}\label{eq:identities}
\begin{equation}\label{eq:expansion}
\begin{split}
  (a+b)^2 &= a^2+2ab+b^2 \\
          &= (a-b)^2+4ab
\end{split}
\end{equation}

A related identity is
\begin{equation}\label{eq:product}
  (a+b)(a-b)=a^2-b^2.
\end{equation}
\end{subequations}

The group is~\eqref{eq:identities}.
Its members are~\eqref{eq:expansion} and~\eqref{eq:product}.

\end{document}

In this standalone example, expect the group reference (1) and member references (1a) and (1b). The intermediate line inside split has no independent number. Save this example as a separate file if you want to compare its numbering with the first document. The distributed examples likewise show a multiline expression receiving one subordinate number. Subnumbered equation examples.

Diagnose a wrong or unresolved reference

Check the symptom before changing counters or typing numbers into the prose. The following sequence separates reference lookup problems from a mismatch between the chosen environment and the intended target.

  1. If the PDF shows ??, compile again. References are read from auxiliary information produced by an earlier run. If the problem remains, compare the spelling of the reference key with its label. LearnLaTeX: Cross-referencing.
  2. If the group reference contains a letter, inspect its label position. Put the parent label immediately after \begin{subequations}. Put each child label on its numbered row before \\, or before \end{align} for the final row. amsmath User’s Guide, §3.11.3.
  3. If separate rows have no separate numbers, inspect the environment. An aligned block inside equation provides one enclosing target. Use the structure in the first example when each row needs a reference. amsmath User’s Guide, §3.7.
  4. If unrelated equations acquire letter suffixes, inspect the wrapper’s end. Everything numbered inside the wrapper participates in its subordinate sequence. Move \end{subequations} to the actual end of the intended group. Subnumbered equation examples.

Use descriptive keys such as eq:system and eq:difference when adapting the examples. The prefix eq: is a naming convention for the author; LaTeX does not use it to decide what kind of object the label identifies. Placement supplies that context. LearnLaTeX: Cross-referencing.

Verify that the references survive an edit

After the initial document compiles, try one controlled edit: insert an ordinary numbered equation before the group. Recompile until reference-change warnings settle, then inspect the group reference, one child reference, and the following equation. In the first example, they should now read (2), (2b), and (3). This is an illustrative check derived from the package’s counter behavior. The amsmath package, §15.

That edit tests the reason to use labels: the referring sentences should continue to identify the intended equations after numbering changes. Keep the parent label at the wrapper’s opening, give every cited child its own numbered target, and use the complete example as a small comparison document when adapting a larger manuscript.

Sources