This review presents a new methodical approach to the classification of dimensionless numbers
as pair relationships of the main forces controlling transport phenomena in fluids at the
macroscopic level by using a tabulated form. To memorize the transfer numbers at the molecular
level a rule in the form of a mnemonic triangle is suggested. The structure of some traditional
dimensional numbers presented as the ratio of more than two forces, or the ratio of a force to a
geometric mean of two other ones, is also discussed. A classification of natural convection as
the interaction of body forces and surface forces, taking into account that sensitive to the force
field the fluid density and the surface tension depends on temperature or composition, is presented.