Tuning line breaks

Inside a justified paragraph, LaTeX optimizes where to break lines automatically. Usually that is enough — but sometimes you want a say: “break here,” or “don’t break here.” This page is not about separating paragraphs (see “Line & paragraph breaks” for that). It is the toolkit for **fine-tuning where lines break *within* a paragraph**: forcing breaks, preventing them, controlling hyphenation, and living with the Overfull / Underfull warnings.

A ground rule first: separating paragraphs (with a blank line) is a different matter from the line-break tuning covered here. A blank line starts a new paragraph, with its indent and inter-paragraph spacing. What follows instead nudges where line *ends* fall within a single paragraph. If that distinction feels fuzzy, read “Line & paragraph breaks” first.

Forcing a line break

To force a break mid-paragraph there are two families of commands. The first is **\\ and \newline. Both merely “end the line here” without leaving paragraph mode. Crucially, they break at the text’s natural length without stretching it** — so the line stops short of the right margin and stays short. This suits addresses or verse, where each line should end exactly where you intend.

The second is **\linebreak**. It breaks *and* stretches the interword spaces so the line justifies to the right margin. So unlike \\ the line is never left short — instead the word spacing may turn gappy. Its strength is set by \linebreak[n] (n an integer 0–4): 4 means “force the break” (the default with no argument), while 1 is a mild “another spot might be better, but here is acceptable.” In short, \linebreak is a *suggestion* about where to break, and the line’s final stretching is left to LaTeX’s justification.

The contrast is obvious once you compile it. Put \\ mid-sentence and the line snaps off there, leaving white space at the right as it moves on. Put \linebreak at the same spot and the line stretches all the way to the right edge, opening awkward gaps between words. The mnemonic: **\\ to break short, \linebreak to justify to the margin.** Note that \\ has extensions — a length in square brackets, as in \\[2ex], adds vertical space after that line, and \\* forbids a page break at that point (see “Line & paragraph breaks”).

% \\ は短く折る／\linebreak は右端まで引き伸ばす
第一の道は険しく\\
第二の道はなだらかだ。

非常に長い一文をここで\linebreak 区切ると、左側の語間が広がります。

In paragraph mode \newline equals \\, but inside a table (tabular) their roles split: **in a paragraph-making cell such as a p{...} column, \newline breaks only within the cell, while \\ breaks the whole row.** Use \newline when you want an address on two lines inside one cell without ending the row. And using \\ where there is no line to end (such as right at the start of a paragraph) raises the error There's no line here to end.

Preventing a line break

For the opposite need — “don’t break here” — there is a matching set of tools. The handiest is the tilde **~ (a “tie”): a non-breaking interword space** used to keep what is on either side on the same line, as in Fig.~3, Dr.~Smith, Chapter~12, or equation~\ref{eq:1}. A ~ is the width of an ordinary interword space; the only difference is that no line break may occur there. Because it still allows the adjacent words to hyphenate, it is gentler than gluing a whole word with \mbox, and is usually the first choice.

To set how strongly you resist a break, use **\nolinebreak[n]**. The mirror of \linebreak, with n from 0 to 4: 4 (the default) completely forbids a break there, while 1 is a mild “please avoid breaking here.” When a whole phrase must not be split, **\mbox{...} is handy: it treats its contents as a single box and neither breaks them across lines nor hyphenates them** (think of keeping T-34 from splitting into T- and 34). \mbox is LaTeX’s equivalent of plain TeX’s \hbox, and writing \hbox{...} directly behaves the same. The low-level **\nobreak** places an infinite “do not break here” penalty at that point, and underlies commands like ~.

A caveat: a word wrapped in \mbox can never break, so if it does not fit it will jut past the right margin and become an Overfull box. Freezing genuinely breakable things — long URLs, chemical names — inside \mbox therefore backfires. Reach for ~ when it is just one space you want to protect, and \mbox only for a short phrase you must not split.

Controlling hyphenation

In Western text, LaTeX automatically performs hyphenation, inserting a hyphen to break a word across lines. It is usually sensible, but can misfire on proper nouns or jargon not in its dictionary. For a one-off fix, use **\- (backslash-hyphen, the discretionary, or optional, hyphen). It embeds permission inside a word — “you may break here if needed**” — so writing man\-u\-script, for example, lets that word break only at the marked points.

A subtlety lurks here. Insert even one \- into a word and **LaTeX will hyphenate it *only* at the points you marked, never at the others it would have found itself.** So \- means not “you may also break here” but “you may break only here.” You must therefore spell out every break point you want. Note that \- does not *force* a break — it merely says “if you break, it must be one of these spots.”

If the same word recurs throughout the document, register exceptions once in the preamble without cluttering the body. List words with their break points marked by hyphens in **\hyphenation{...}. This is a global setting that applies to the whole document, and multiple declarations accumulate. Two caveats: use lowercase letters, and TeX matches exactly** — inflected forms (plurals and the like) are treated separately. Where \- is “just here, just now,” think of \hyphenation as “a dictionary entry for the whole document.”

% プリアンブルで分割例外を一括登録（小文字・分割点をハイフンで）
\hyphenation{man-u-script analy-sis FORTRAN}

% 本文では、その場かぎりの任意ハイフン
super\-cali\-fragi\-listic

Conversely, to forbid hyphenating a particular word entirely, wrap it in **\mbox{...}**: a box’s contents never break, so no hyphen is inserted. As noted above, though, an unbreakable word can overrun the line and cause an Overfull box. Japanese text has no hyphenation in this sense, so \- and \hyphenation are essentially tools for the Western-language portions.

Overfull / Underfull \hbox warnings

At compile time you have likely seen **Overfull \hbox or Underfull \hbox** warnings. An \hbox (horizontal box) is a single typeset line. Overfull means “the line did not fit and stuck past the right margin,” while Underfull means “the line is too loose, with stretched interword spacing.” Knuth chose to warn rather than silently accept substandard typesetting, leaving the fix to you. The draft class option prints a black rule beside each overfull line so you can spot it.

Why does LaTeX leave a line poking past the margin? Because when TeX cannot find a “presentable” set of breaks within the stretch it is allowed, it shows the overflow as a warning rather than spacing the line out grotesquely. The usual culprit is a long word, URL, or proper noun that will not break. The first things to try are the local fixes from the sections above: give the word break points with \-, drop a \linebreak nearby, or untangle a space with ~. Often that settles it.

When local fixes are tedious and you would accept some looseness to cut down the warnings, switch to the blunt tools. The **\sloppy** declaration greatly relaxes the break tolerance (internally setting \tolerance to 9999), accepting loose interword spacing in exchange for avoiding overfull boxes. The default is **\fussy**, which prioritizes even spacing at the cost of an occasional overfull box. After issuing \sloppy, return to \fussy at the end of the stretch you wanted to affect.

These declarations have a quirk, though. Line breaking obeys whichever declaration is in force when the paragraph ends, so to make \sloppy affect a given paragraph you must include the blank line or \par that ends it within \sloppy’s scope. To confine the effect to a single paragraph, the **sloppypar environment** is safer: only the text between \begin{sloppypar} and \end{sloppypar} is set as if under \sloppy, and normal behavior resumes on exit.

A more refined approach is **\emergencystretch** — e.g. \setlength{\emergencystretch}{3em} — which permits extra stretch only as a last resort, trimming overfull boxes without disrupting spacing as bluntly as \sloppy. The microtype package, which subtly expands or contracts glyphs to fit the margin, also helps. The table below sums up each command’s role.