To continue a bracketed LaTeX expression across lines, use fixed-size brackets or pair each line’s visible bracket with an invisible delimiter. For one tall pair around the entire block, put an inner multiline environment inside the brackets.

This guide helps you choose the layout and edit a complete LaTeX document on a Mac. All expressions below are synthetic algebraic examples. The distinction that matters is where you want the brackets to appear: beside the first and last terms, or along the sides of the whole block.

Which bracket layout should you choose?

Use this decision aid before changing the source. The environment choices follow the display structures documented in the amsmath user’s guide, sections 3.3–3.7 and the mathtools manual, section 3.4.2.

Intended result Suitable structure Bracket method
Opening bracket on the first line; closing bracket on the last equation containing split Matching fixed sizes, such as \Bigl[ and \Bigr]
Same placement, with automatic sizing on each line equation containing split \left[ ... \right. followed by \left. ... \right]
One tall pair enclosing aligned rows equation containing aligned Put \left[ before aligned and \right] after it
One tall pair enclosing a staggered continuation equation containing multlined Load mathtools; put the brackets outside multlined

For a long sum, start with the first option. For a compact expression that should look like one bracketed object, try the third. This is an editorial choice about how the reader should follow the expression; the examples below let you compare those choices without changing its algebraic content.

How do you continue an expression with fixed-size brackets?

The amsmath guide explicitly recommends fixed-size delimiters as a solution when a line break would interrupt a \left–\right pair. Its size families include \bigl/\bigr, \Bigl/\Bigr, \biggl/\biggr, and \Biggl/\Biggr. See sections 4.14.1 and 10.2 of the guide.

Here is a complete document. The expression opens its square bracket on the first row and closes it on the second, leaving the continuation point free of visible brackets.

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{equation}
\begin{split}
F(x) &= \Bigl[a_0+a_1x+a_2x^2+a_3x^3 \\
     &\qquad {}+a_4x^4+a_5x^5\Bigr].
\end{split}
\end{equation}

\end{document}

In this example, the first & establishes the alignment before the equals sign. The second row begins with the corresponding &, then an indentation. Both visible square brackets use the same size family. There is no closing bracket before the break and no opening bracket after it.

The empty group in {}+ prints nothing; it preserves binary-operator spacing for the continuation plus sign. Keep the sign when moving terms between rows so that the displayed expression retains the intended sum.

To adapt this example, replace the polynomial terms first, leaving the structural commands in place. Then inspect whether the brackets suit the tallest term on either row. If you introduce a fraction, consider a larger matching pair. Change both ends together rather than adjusting just the opening bracket.

How do invisible delimiters work across the break?

A \left–\right pair cannot cross a row break or alignment separator directly. The documented workaround closes each segment with a null delimiter: \right. or \left.. The dot selects an invisible delimiter; it does not print a period. See the amsmath guide’s explanation of the missing-right-delimiter error.

This complete example uses a fraction to make delimiter sizing relevant:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{equation}
\begin{split}
G(x) &= \left[\frac{a_0+a_1x}{1+x^2}+a_2x^2\right. \\
     &\qquad \left.{}+a_3x^3+a_4x^4
       \vphantom{\frac{a_0+a_1x}{1+x^2}}\right].
\end{split}
\end{equation}

\end{document}

Read the bracket structure row by row. The first row has a visible opening bracket and an invisible closing delimiter. The second has an invisible opening delimiter and a visible closing bracket. Each row therefore contains a complete delimiter pair.

How do you keep the visible brackets the same height?

Separate automatic pairs can choose different sizes. The guide recommends \vphantom with the tallest material from the other segment to equalize them. In this example, the second row repeats the fraction inside \vphantom; the fraction itself remains invisible. See amsmath section 10.2.

Treat that phantom as part of the layout maintenance. If you replace the first-row fraction with taller material, update the phantom too. If that becomes cumbersome as you revise the expression, return to matching fixed-size brackets and assess their appearance in the PDF.

How do you put one tall bracket pair around all the lines?

The amsmath package provides aligned as an inner building block that can sit inside a larger expression. Put the bracket commands outside it. The row breaks then belong to the enclosed block. This use of inner environments is documented in amsmath section 3.7.

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{equation}
H(x)=\left[
\begin{aligned}
  &a_0+a_1x+a_2x^2 \\
  &{}+a_3x^3+a_4x^4
\end{aligned}
\right].
\end{equation}

\end{document}

Here, the visible brackets surround the combined height of the two rows. Compare that with the earlier examples, where each bracket belongs beside one row. Choose this layout when you want the bracketed sum to read as a single stacked component, perhaps beside another factor.

When should you use mathtools and multlined?

For a staggered block, mathtools supplies multlined: its first row starts at the left and its last finishes at the right. It also supports an optional width. The package loads amsmath automatically. See the mathtools manual, sections 2 and 3.4.2.

\documentclass{article}
\usepackage{mathtools}

\begin{document}

\begin{equation}
J(x)=\left[
\begin{multlined}[c][0.65\linewidth]
  a_0+a_1x+a_2x^2 \\
  {}+a_3x^3+a_4x^4
\end{multlined}
\right].
\end{equation}

\end{document}

The width 0.65\linewidth is an illustrative layout choice. Adjust it after viewing your own expression, allowing room for the left-hand side, brackets, and equation number. Keep the explicit break at a sensible algebraic boundary.

The similar names identify different environments: multline is an outer amsmath display, while multlined is a mathtools inner block. Use multlined for the nesting shown here. Its manual also records limitations when nested inside arrays or matrix variants; this example uses it directly inside equation. See mathtools section 3.4.2.

How do you check the result on a Mac?

These examples assume a working LaTeX installation and editor. TeXShop’s default configuration uses PDF-producing TeX programs, and its Help menu includes the TeXShop Manual. See the developer’s About TeXShop page.

  1. Copy one complete document above into a new TeXShop source document and save it as brackets.tex.
  2. Typeset it using the LaTeX engine. TeXShop supports initiating typesetting with Command-T, as documented in its official changes document.
  3. Inspect the PDF for the chosen bracket placement, matching heights, and a readable continuation sign.
  4. Replace the sample terms with your expression, typeset again, and check the widest row against the available space.

The examples were inspected against the package documentation; they were not compiled for this article because a LaTeX compiler was unavailable in the research environment.

If compilation stops, first check whether you accidentally left a visible \left[ on one row and its \right] on another. Next compare the environment names and package line with the complete example. If compilation succeeds but the layout feels crowded, move the break before changing bracket sizes.

For most continued sums, matching fixed-size brackets give a straightforward starting point. Use invisible delimiters when row-specific automatic sizing is useful, and an enclosing aligned or multlined block when you want tall brackets around the complete stack. Finish by checking the PDF with your actual terms, since their height and width determine the final layout.

Sources