Intercolumniation
Vitruvius III.3: five ways of spacing columns, and why wider spacing asks for stouter columns.
The rules it illustrates
The five intercolumniations. Columns may be spaced 1½ diameters apart (pycnostyle), 2 (systyle), 2¼ (eustyle), 3 (diastyle) or more (araeostyle). The eustyle, with a wider central bay of 3 diameters, is the best for use and beauty.
Where it appears in the book
In the chapters What Is Architecture?, The Tuscan Order, Columns in Company.