Sometimes an existing collection of LATEX 2.09 documents makes it inconvenient or impossible to abandon the old commands entirely. If this is the case, then it is possible to accommodate both conventions by making special provision for documents processed in compatibility mode.
\if@compatibility
This switch is set when a document begins with \documentstyle
rather
than \documentclass
. Appropriate code can be supplied for either
condition, as follows:
\if@compatibility <code emulating LaTeX 2.09 behavior> \else <code suitable for LaTeX2e> \fi