How to align equations in LaTeX

To line several equations up on their equals signs, LaTeX has the align environment of the amsmath package: put & just before the symbol to align on, and \\ at the end of each row. This page covers align and the three environments that go with it — gather, multline and aligned — with the source to copy.

Aligning on the equals sign

(a+b)^2 &= (a+b)(a+b) \\
        &= a^2 + 2ab + b^2

Each & marks the point that is lined up from one row to the next. The second row has nothing on its left: it starts with &=.

In a .tex file these rows go between \begin{align} and \end{align}. In Texifine you type the rows only: insert an Equation block and choose align in the right column.

Several equations, one under the other

The same environment sets a system:

x + y &= 5 \\
2x - y &= 1

A comment on each row

A second && opens another aligned column, which is where a short justification goes. \text{…} writes words inside a formula.

x &= y + z && \text{by definition} \\
  &= 2z    && \text{since } y = z

Numbering

align numbers every row. The starred form, align*, numbers none.

In Texifine the choice is Numbered or Plain in the right column, for the whole block: every row or none. A numbered block of three rows shows its range, for example (2)–(4). See Numbering and references.

For one number on a group of aligned rows, use aligned (below).

Centred, not aligned: `gather`

When the formulas have nothing to line up on, gather centres each of them. No &, only \\ between rows.

a^2 + b^2 = c^2 \\
e^{i\pi} + 1 = 0

One long formula over several lines: `multline`

multline is for a single formula too long for the line: the first line is set to the left, the last to the right, and the whole takes one number. \\ marks where it breaks.

p(x) = a_0 + a_1 x + a_2 x^2 + a_3 x^3 + a_4 x^4 \\
  + a_5 x^5 + a_6 x^6 + a_7 x^7 + a_8 x^8

One number for the whole group: `aligned`

aligned is written inside another formula. The rows are aligned as in align, but the group is one object: it takes a single number, centred on its height.

\begin{aligned}
  \nabla \cdot \mathbf{E} &= \frac{\rho}{\varepsilon_0} \\
  \nabla \cdot \mathbf{B} &= 0
\end{aligned}

In Texifine, keep the block's environment on equation and type the aligned group yourself.

Because it is an object, it can also sit next to something else — a brace, for a system:

\left\{
\begin{aligned}
  x + y &= 5 \\
  2x - y &= 1
\end{aligned}
\right.

\left\{ draws a brace as tall as the group; \right. closes it with nothing.

Which one to choose

You want	Environment
Steps of a calculation, aligned on `=`	`align`
Several unrelated formulas, centred	`gather`
One formula too long for the line	`multline`
Aligned rows that share one number	`aligned`, inside `equation`
A single formula	`equation`

Common mistakes

An & in an environment that does not align. In equation, LaTeX stops on "Misplaced alignment tab character &"; in gather or multline, on "Extra alignment tab has been changed to \cr". Choose align, or wrap the rows in aligned.
A missing \\. Two rows without \\ between them are read as one row.
A blank line in the source. In a .tex file, an empty line inside align stops the compilation on "Paragraph ended before \align was complete". Texifine removes the empty lines of a formula before compiling, so one left by accident does no harm.

When a document does not compile, the PDF tab shows the first error and its line. See Fixing compilation errors.