-logy is a suffix in the English language, used with words originally adapted from Ancient Greek ending in -λογία (-logía).
[2] The suffix became productive in English from the 18th century, allowing the formation of new terms with no Latin or Greek precedent.
[8] In English names for fields of study, the suffix -logy is most frequently found preceded by the euphonic connective vowel o so that the word ends in -ology.
However, when new names for fields of study are coined in modern English, the formations ending in -logy almost always add an -o-, except when the root word ends in an "l" or a vowel, as in these exceptions:[10] analogy, dekalogy, disanalogy, genealogy, genethlialogy, hexalogy; herbalogy (a variant of herbology), mammalogy, mineralogy, paralogy, petralogy (a variant of petrology); elogy; heptalogy; antilogy, festilogy; trilogy, tetralogy, pentalogy; palillogy, pyroballogy; dyslogy; eulogy; and brachylogy.
As with other classical compounds, adding the suffix to an initial word-stem derived from Greek or Latin may be used to lend grandeur or the impression of scientific rigor to humble pursuits, as in cosmetology ("the study of beauty treatment") or cynology ("the study of dog training").