Meaning of '-onomy', '-ology' and '-ography'

The suffix -logy means a branch of learning, or study of a particular subject.

The suffix -nomy means a system of rules or laws, or body of knowledge of a particular subject.

These two are often intertwined as you might expect.

(Note that -ology and -onomy are alternate forms which include the connecting vowel -o-.)

The suffix -graphy refers to something written about a particular subject.

It's no coincidence that -logy- appears to be so common, as it refers to a wide variety of branches of study.

Another familiar suffix is -metry which refers to measurement, e.g. geometry, to continue your geo- theme.