What is the use of percent signs (%) at the end of lines? (Why is my macro creating extra space?)

Question

I see that the code in many packages and examples contains percent signs % at the end of (many) lines. What are they used for? Do they affect the parsing of those lines?

Answer 1

The short answer is what others have said, % starts a comment that goes to the end of the line. The normal effect is that it doesn't insert the space (or a \par) from the newline.

The longer answer is that as TeX parses its input, it reads the input file line by line. It strips off tailing whitespace (including any carriage return and newline) and then appends a character corresponding to the number in the \endlinechar register (provided it isn't -1 or greater than 255). By default, the value is 13 which corresponds to an ASCII carriage return. (TeX denotes this by ^^M.) You can change this register to be any value.

Unless the category codes have been changed (e.g., by the \obeylines macro), ^^M has category code 5, or end of line. When TeX reaches an end of line character, it discards the rest of the line and does one of three things: nothing, insert a space token, or insert a \par token, depending on what state TeX was in (S, M, or N, respectively).

So what does this have to do with %? Well, since the comment character causes TeX to ignore the rest of the input line, the end of line character that got appended gets ignored too.

This can frequently be important when playing around with category codes of ^^M (again, using \obeylines or similar).

The long answer is contained in Chapter 8 of The TeXbook.

One final use that no one has mentioned is that it is sometimes necessary for the line to end with a space character and not a end of line character. One example is that a backslash followed by a space is different than a backslash followed by a newline:

\show\ 
\show\ %

In the first line, there's a space following the \, but it'll get stripped off as described so what you get instead \^^M as you can see by what TeX prints out.

> \^^M=macro:
->\ .

That is, \^^M is a macro that expands to a control space: \ . In the second case, the comment prevents the space from being stripped off and the end of line char is ignored. TeX prints out the following.

> \ =\ .

That is, \  is a TeX primitive (see control space in either The TeXbook or TeX by Topic).

The usual reason for % is suppressing spaces in macro definitions. Consider the macros \nopercents and \percents below.

\documentclass{minimal}
\newcommand*\bracket[1]{[#1]}%
\newcommand*\nopercents[1]{
    \bracket{#1}
}
\newcommand*\percents[1]{%
    \bracket{#1}%
}
\begin{document}
X\nopercents{blah}X

X\percents{blah}X
\end{document}

Superficially, they appear to do the same thing: pass their input to \bracket. The difference is that the newlines becomes space tokens in \nopercents but are ignored in \percents due to the %. So X\nopercents{blah}X expands to X [blah] X whereas X\percents{blah}X expands to X[blah]X.

Addendum regarding spaces at the beginning of a line.

A % only swallows whatever follows it on a line. It does not have any effect on white spaces that begin the next line. Under most circumstances, spaces at the beginning of a line are ignored by TeX itself. There are a couple of exceptions:

• When the line is otherwise entirely blank, it is interpreted as a paragraph break.

• When \obeyspaces is in effect, every space is carried into the output; this is true within verbatim mode and can also be requested explicitly.

If \obeyspaces is in effect while a command or environment is being defined, if the definition occupies more than one line, any spaces at the beginning of a continuation line will be preserved in the definition. Indentation is often used in defining commands or environments to make the code easier to understand (usually a good thing), but while \obeyspaces is in effect, this has an unwelcome result and should be avoided. \obeyspaces should normally be used only within a delimited scope ({ } or \begingroup ... \endgroup) to avoid unwanted results.

Answer 2

You already got lots of answers. You can also just experiment yourself:

\documentclass{article}
\begin{document}
Hello%
world!
\end{document}

Try compiling this with and without the %. Then you see yourself that the % makes the space produced by the newline disappear. (Note that you'll still get the space if you write Hello % with a space before the % – try it out!) All the details are given in TH's great answer.

Answer 3

A percent sign, %, allows to end a line without generating a space character -- very useful when writing macros.

Answer 4

When I was starting out with TeX, I have read many times that the percent sign "swallows" all the whitespace after it. Whitespace includes spaces, tabs, and linebreaks.

Answer 5

When % is not the answer

1. In the middle of a control sequence.

   \ta%
   u

will not be reconstituted into \tau. A control sequence must be input as an unbroken unit.

2. After a number or <dimen> that is being used in an assignment. TeX will continue reading until the next token is definitely not numeric, or in the case of a <dimen> not plus or minus. Use \relax instead.

These exceptions are already covered, but "buried", so a more prominent reminder is warranted.

Another possibility, not as easily identified and often not mentioned in user documentation, is insertion of "padding" by a package, as in this example.

\documentclass{article}
\usepackage{tcolorbox}
\newcommand{\pink}[1]{\colorbox{red!20}{#1}}
\newcommand{\Pink}[1]{{\fboxsep=0pt\colorbox{red!20}{#1}}}
\begin{document}
compress \par
com\pink{p}ress \par
com\Pink{p}ress
\end{document}

This isn't usual, but action by a package should be considered when more likely possibilities fail to cure the problem.

Answer 6

The question of the use of the symbol % is dealt with in numerous TeX.SE questions (see a non-exhaustive list at the end), so this answer aims to make a summary of the practical recommendations concerning the use of the percent symbol.

Outline

1. How TeX "deals with spaces"?
2. Practical recommendations concerning the use of the percent symbol
3. Related posts
4. Other resources

1. How TeX "deals with spaces"?

The rules applied to spaces (except inside verbatim/math mode or when \obeyspaces is in effect) are listed in this Martin Scharrer's answer.

For deeper explanations and "traps" regarding how TeX "deals with spaces":

• Mico's answer (for beginners this answer is quite complete, very pedagogical and plenty of practical examples).
• TH.'s answer here.
• egreg's answer (introducing also the three TeX modes: horizontal, vertical or math).
• another egreg's answer introducing the use of \relax, \ignorespaces and also the issues related to spaces after a constant number (and much more!).
• This blog introducing the different states of TeX when reading input.
• Chapter 8 of The TeXbook.

2. Practical recommendations concerning the use of the percent symbol

1. Use a % when you want TeX "to exclude everything that's on the remainder of the current input line from further processing" (including the "invisible" end-of-line character), useful for writing a comment for e.g.

2. No need to use % sign in preamble (except in macro definition, see below). So NO need of % after the closing } of a macro definition.

3. No need to use % sign between two consecutive undelimited arguments (details here): Then \parbox {3cm} {text} is equivalent to

   \parbox
     {3cm}
     {text}
   

4. In macro definition [details here]:
   A % will prevent from possible extra spaces before AND after the word bar in the following example (unless the macro is used in math mode):

   \newcommand{\foo}{%
     bar%
   }
   

   Which is equivalent to \newcommand{\foo}{bar} OR:

   \newcommand{\foo}{%
     bar}
   

   But it is not optimal because in some cases (depending on the context at the macro "call"), there will be no spurious spaces "added" before and/or after the word bar in the output, then the % signs will be unnecessary (but not problematic).

   So, if you really wants to optimise/minimise the use of the % sign in your macro definition, then take a look at this egreg's answer and see also ShreevatsaR's answer for good practice regarding macro writing.

5. Never put a % sign just after a constant number but always PUT a space or a new line or a \relax instead (for e.g. - hereafter the _ symbol stands as a space character - do \ifnum\mycounter<13_ or \mycount=123_ or \mycount=123\relax but NOT \mycount=123%), details here and here.

   • About the use of \relax (especially in "incomplete glue assignment") an possible issues see this answer and also this one.

6. Put a % between two consecutives \end{subfigure} and \begin{subfigure} or \end{minipage} and \begin{minipage} if you want to avoid undesired space (which could induce non aligned subfigure or minipage, e.g. and detailed explanation here) BUT an \hfill will do the trick (even "better/cleaner") also (see here).

7. No need to put a % after a control sequence (e.g. \foo) because the space is already ignored.

8. In LaTeX3 syntax, spaces are ignored in macro definition unless you explicitly specify the space with a ~ character (see Leo Liu's answer).

3. Related posts

Legend : date in mm/dd/yyyy format.

#	Question	Answer/comment	Insight
1	Where are the necessary places to be appended with % to remove unwanted spaces? [duplicate] 06/05/2011	Leo Liu's answer	Overview of different way to "deal" with space in macro def
2	//	egreg's answer (cited in §1)	Introducing the use of \relax, \ignorespaces and also the use of spaces after a constant number and undelimited arguments
3	Tex Capacity Exceeded (if remove % after use of macro) 06/13/2011	Ryan Reich's answer	Illustrative problem with spurious spaces into foreach loop, main rules applied to spaces presented + some maro definition examples
4	When is it harmful to add percent character at end of lines in a \newcommand, or similar 11/14/2011	egreg's answer (cited in §1)	Introducing the three TeX modes (horizontal, vertical or math) and related effect on space tokenization, the use of spaces after a constant number and exercises
5	//	Martin Scharrer's answer (cited in §1)	Listing the rules applied to spaces
7	Why the end-of-line % in macro definitions? [duplicate] 01/13/2012	Martin Scharrer's answer	Differentiating macro with and without argument and the use of braces {} AND notice that the "(La)TeX rules can be dynamically changed"
8	Unwanted spaces [duplicate] - 04/18/2012	egreg's answer	Spurious spaces caused by missing % in macro def
9	Do assignments need to be appended by % to remove the trailing white spaces? 04/09/2013	egreg's answer (cited in §2)	Focus on the use of % sign after register, parameter, constant number, and focus on \relax
10	//	Ryan Reich's answer	Explanation of how TeX scan "a dimension" or "a register"
11	//	Joseph Wright's answer	A bit of details about tokenization of macros
12	Does the comment symbol % remove the carriage return at the end of the line?	egreg's answer (cited in §2)	Description of the tokenization process and examples where % after a control sequence can be harmful.
13	Unwanted spaces in user defined structures and the uses of % [duplicate] 03/16/2018	Steven B. Segletes' answer	Details about the use of \relax after constant number and possible issues AND "indentation and vertical mode" AND "brackets [ ] and Optional Arguments"
14	//	ShreevatsaR's answer	Good practice regarding macro writing in order to avoid spurious spaces
15	How to compare number from \newenvironment argument 11/14/2018	Phelype Oleinik's answer	Practical example on the usage of % and ifnum
16	(La)TeX -- What does the '%' character do? [duplicate] 10/13/2020	Mico's answer (cited in §1)	Interesting answer for beginners: presentations of the main rules regarding spaces, % usage in macro and after constant number, illustrate by practical examples

4. Other resources

#	Resource	Remark	Source
1	The TeXbook, Chapter 20	"For general knowledge of macro definitions"	Yan Zhou's answer
2	TeX by Topic "distributed with TeXLive"	"Good introduction to how TeX read in your input"	Yan Zhou's answer
3	egreg's article, "Recollections of a spurious space catcher", TUGboat, Volume 36 (2015), No. 2), p.149-161		ShreevatsaR's answer
4	egreg's video, "Recollections of a spurious space catcher"		ShreevatsaR's answer