\textit is to \emph as \textbf is to...?
The idea behind \emph
is that it provides a high level way for giving emphasis to a part of the text. High level in the sense it is “independent” of the actual implementation.
The default behavior of \emph
is to use italics when in an upright context and upright when in an italics context, but this can be modified on a document’s basis (or by a package implementing a particular style). In particular, your question has no real answer: \emph
and \textit
bear no “abstract” relationship; the relationship is only at the default implementation level.
This is different from stating some part of text is in italics; for instance, theorems are commonly typeset in italics and the styles use \itshape
for this, not \em
(the declarative form of \emph
). Similarly they use \bfseries
for the theorem tag (or \scshape
or whatever).
You're free to define as many similar commands as you want. If your style requires a sort of “strong emphasis”, you can define \strong
as you please, maybe using \bfseries
in normal context and \extrabfseries
(if your font supports it, the name is hypothetical) in a \strong
context. Before doing this, think deeply whether your readers will be able to appreciate the difference between \emph{word}
and \strong{word}
(which I think they won't).
There may be some confusion as to how \emph
and \em
are defined. From the latex kernel (cf latex.ltx
):
\DeclareTextFontCommand{\emph}{\em}
\DeclareRobustCommand\em
{\@nomath\em \ifdim \fontdimen\@ne\font >\z@
\eminnershape \else \itshape \fi}%
\def\eminnershape{\upshape}%
Basically, \emph
(via \em
) checks if it's already in "emphasis mode". If it's not, \itshape
is executed; and if it is, \eminnnershape
-- which, by default, is set to \upshape
-- is executed. (\eminnershape
can be modified via a \renewcommand
instruction. E.g., running \renewcommand\eminnershape{\scshape}}
would select small-caps for "inner" emphasized material.)
So it's not the case that \emph
is just "more semantic" than \textit
. Indeed, \emph
is more semantic -- it lets you choose the method of typographic emphasis (the defaults are \itshape
for "outer" and \upshape
for "inner" material) -- but it also provides a way of providing emphasis within emphasized material.
Finally, just in case you're curious how \DeclareTextFontCommand
is defined in the LaTeX kernel, here goes:
\def \DeclareTextFontCommand #1#2{%
\DeclareRobustCommand#1[1]{%
\ifmmode
\nfss@text{#2##1}%
\else
\hmode@bgroup
\text@command{##1}%
#2\check@icl ##1\check@icr
\expandafter
\egroup
\fi
}%
}