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

4.4. Generalized lists 75
However, the label is often required to contain a sequential numeration.
For this purpose, a counter must be created with the \newcounter{name}
command, where name is its designation. This command must appear
before the first application of the counter in a list environment. Suppose
a counter named marker has been defined for this use, then the argument
stnd lbl could be any of the commands for printing counters described in
Section 4.3.5: for example, \arabic{marker} produces a running Arabic
number.
Even more complex labels can be made up in this way. If the sequential
labels are to be A–I, A–II, . . . , stnd lbl is set to A--\Roman{marker}.
Before a counter can function properly within the standard label, it
must be associated with that list by including in the list decl the command
\usecounter{counter}, where counter is the name of the counter to be
assigned (marker in the above example).
The standard label is actually generated by the command \makelabel{label},
! which is called by the \item command. The user can redefine \makelabel with
the aid of the \renewcommand in the list declaration:
\renewcommand{\makelabel}{new definition}
If the standard label is defined in this manner, the corresponding entry in the
list environment is left blank. This is because \makelabel is the more general
command and overrides the other definition.
4.4.2 List style parameters
There are a number of style parameters used for formatting lists that
are set by L AT E X to certain standard values. These values may be altered
by the user in the list decl for that particular list. The assignment is
made in the usual way with the \setlength command. However, if the
assignment is made outside the list environment, in most cases it will
simply be ignored. This is because there are preset default values for
each parameter at each level that can only be overridden by list decl.
The style parameters are listed below and are also illustrated in Figure
4.1 on the next page, which is based on one taken from Lamport (1985,
1994).
\topsep
is the vertical spacing in addition to \parskip that is inserted
between the list and the enclosing text above and below. Its
default value is set at each list level and cannot be globally
redefined outside the list decl.
\partopsep
is the vertical spacing in addition to \topsep + \parskip that
is inserted above and below the list when a blank line precedes
the first or follows the last \item entry. It may be redefined
globally, but only for the first and second levels.