While Christaller's model is rarely found, it still provides a useful rule for the establishment of towns, called central place theory.

Being a collection of the most useful rules in arithmetic.

An apprentice cannot do better than keep this useful rule in mind.

This view is simplistic, but it is a useful rule of thumb.

There are some exceptions to Bancroft's rule, but it's a very useful rule of thumb for most systems.

(Another useful rule he imparts to me: Post in the morning, when you're no longer drunk.)

If it's lucky, it can end up associating a useful rule with a random bit of text.

The general bad consequence is, the violation of some necessary or useful general rule.

Other matters receive far less attention, and useful rules remain hollow phrases.

From the Kolmogorov axioms, one can deduce other useful rules for calculating probabilities.