Guide to LaTeX (4th Edition) (Tools and Techniques

80 Chapter 4. Displayed Text
These are set internally by the parameters \leftmarginn, where n stands for i,
ii, iii, iv, v, or vi. These values can be changed by the user; for example, by
declaring \setlength{\leftmarginiv}{12mm}, the left margin of the fourthlevel
list is shifted 12 mm from that of the third. These declarations must be
made outside of the list environments and not in the list decl.
At each level of list nesting, the internal macro \@listn (n being i to vi) is
called. This sets the value of \leftmargin equal to that of the corresponding
\leftmarginn, unless \leftmargin is explicitly declared in the list environment.
That is, there does not exist a single standard value for \leftmargin
externally, but rather six different ones. The parameter \leftmargin has meaning
only within a list environment.
4.5 Theorem-like declarations
In scientific literature one often has text structures like
Theorem 1 (Balzano–Weierstrass) Every infinite set of bounded points
possesses at least one maximum point.
or
Axiom 4.1 The natural numbers form a set S of distinct elements. For any
two elements a, b, they are either identical, a = b, or different from one
another, a ≠ b.
Similar structures frequently appear with names such as Definition,
Corollary, Declaration, Lemma, instead of Theorem or Axiom. What they
have in common is that a keyword and a running number are printed in
bold face and the corresponding text in italic.
Of course, these could be generated by the user by explicitly giving the
type styles and appropriate number, but if a new structure of that type is
later inserted in the middle of the text, the user would have the tedious
job of renumbering all the following occurrences. With the command
\newtheorem{struct type}{struct title}[in counter]
L AT E X will keep track of the numbering automatically. Here struct type is
the user’s arbitrary designation for the structure, while struct title is the
word that is printed in bold face followed by the running number (for
example, Theorem). If the optional argument in counter is missing, the
numbering is carried out sequentially throughout the entire document.
However, if the name of an existing counter, such as chapter, is given for
in counter, the numbering is reset every time that counter is augmented,
and both are printed together, as in Axiom 4.1 above.
The predefined structures are called with the command
\begin{struct type}[extra title] text \end{struct type}