Is "double-lowercase-L" a ligature (Computer Modern Italic)?

Question

Consider the following MWE:

\documentclass{minimal}

\begin{document}

Which kerning do you prefer?


\bigskip


\emph{well willow wallow wellington} %
\verb+  <--- Version 1, default kerning+


\bigskip


\newcommand{\donothing}{\hspace{0pt}}

\emph{wel\donothing l wil\donothing low wal\donothing low wel\donothing lington} %
\verb+  <--- Version 2, Double-lowercase-L's are different+

\end{document}

On my system (MiKTeX 24.4 + TeXWorks 0.6.9, Windows 11, using LuaLaTeX) the two versions give noticeably different output. The line marked "Version 1" has much wider spacing between the double "Italic lowercase-L" characters, than we see in "Version 2". Here is an image of my output:

And, a second image where we zoom in on the italic part:

Personally I prefer the output in Version 2: it makes the "ill" and "lli" combinations more consistent, plus it just "looks better" to my eye, though I am no expert.

(Adding the \donothing macro in other places, like between the "w" and the "e", has no effect. It's only between the "adjacent l"s that it affects the output. I can also use the same \donothing macro to break ligatures in other examples, like \emph{dif\donothing ferent waf\donothing f\donothing les}, though in that example I would certainly prefer to leave the ligatures intact.)

Questions:

1. Is "double-italic-lowercase-L" a ligature? I never see it included in lists of ligatures, and it seems that it is not so easy to check.
2. If it is a ligature, does it mean that the default kerning ("Version 1") was an intentional design choice? Is it considered "better" than Version 2?
3. If it is not a ligature, why does the \donothing macro alter the output?

Answer 1

This a postscript on egreg's answer.

You're confusing ligatures with kerning pairs.

A ligature is a single character¹ which automatically replaces a consecutive series of two or more input characters.

• f+f -> the single character ff
• ff+i -> the single character ffi
• -+- -> the single character used to indicate a range of numbers
• --+- -> the single character used for the longer dash sometimes used to set a clause apart, for example (but -- is also used for this).

Here's an entry for f_f (which is the name of ff in the postscript font) from an intermediate file used in creating a TeX font:

   (STOP)
   (LABEL D 27) (COMMENT f_f)
   (LIG D 105 D 30) (COMMENT i f_f_i)
   (LIG D 108 D 31) (COMMENT l f_f_l)
   (STOP)

A kerning pair is a pair of discrete characters in a font for which some adjustment to horizontal spacing is specified. This might be positive or negative. Kerning pairs are highly font dependent, although certain combinations are more common than others.

• V+a commonly gets a negative adjustment as the lowercase a can nestle under the arm of the uppercase V to reduce the otherwise gappy-looking space.

Here's an entry from the same file for a kerning pair:

   (STOP)
   (LABEL D 108) (COMMENT l)
   (KRN D 118 R -0.019) (COMMENT v)
   (STOP)

So we have a negative kern specified for a single letter combination for l, though other characters have far more extensive listings.

\donothing prevents both ligature formation and kerning. So the results typeset the ll differently not because this is a ligature, but because \donothing blocks the usual adjustment specified for the kerning pair l+l.

The 'standard' way (I think) to stop the formation of a ligature is to insert \/ as in shelf\/ful where the f+f ought not be replaced by ff.

¹As far as TeX is concerned. It doesn't actually have to be a single character because it may be constructed from two or more other characters or in some other way.

Answer 2

No, there is no “ll” ligature. The font designer (D. Knuth) decided to add some positive kerning between consecutive l's in cmti10:

(CHARACTER C l
   (CHARWD R 0.255554)
   (CHARHT R 0.694445)
   (CHARIC R 0.103334)
   (COMMENT
      (KRN C l R 0.051111)
      )
   )

Personal opinion: I find your unkerned version really bad. Not that I particularly like Computer Modern Italic, though.

Why \donothing suppresses kerning? Because of TeX rules: after the first l you insert a zero width glob of glue. Glue or explicit kerns will disallow implicit kerning defined in the font.

Answer 3

The only fonts I know of that provide an ll-ligature are EB Garamond and Garamond Premier Pro -- but only in the italic font shape, and only if the Ligature=Rare option is employed in a suitable \setxxxxfont directive. (Garamond Premier Pro is distributed automatically with MacOS; I don't know if it's distributed with other operating systems.)

I believe ll is used as a digraph in Spanish, in words such as llamar and pollo. (In Mexican Spanish, and possibly other Spanish variants, the ll digraph is pronounced like a voiced y in English -- as in yes and yellow. See @egreg's comment below for further pronunciation possibilties.)

% !TEX TS-program = lualatex
\documentclass{article}

\usepackage[spanish]{babel} % optional
\usepackage{fontspec} % for '\setmainfont' directive
\setmainfont{EB Garamond}[Ligatures={Common,Rare}]

\begin{document}
se llamar 

\textit{se llamar, se l\kern0pt lamar}
\end{document}