Affixes

Orthodox literally means having the correct opinion (Greek doxa, opinion); orthoepy (Greek epos, epe‑, word) is the correct or accepted pronunciation of words, or the study of it; orthodontics (Greek odous, odont‑, tooth) is the treatment of irregularities in the teeth and jaws; something orthogonal (Greek gōnia, angle) involves right angles.

In chemistry, the form indicates substitution at two adjacent carbon atoms in a benzene ring (orthodichlorobenzene, ortho-aminoanisole), as opposed to the other two possible positions, meta‑ and para‑¹. It is often abbreviated to o‑: o‑tolidine, o‑hydroxybenzaldehyde.

The form can also denote a compound in the highest state of hydration, containing the maximum number of hydroxyl groups: an example is an ortho acid such as orthosilicic acid. Such acids are often unstable and lose water to form compounds designated by meta‑, as orthophosphoric acid becomes metaphosphoric acid on heating.