In printed dictionaries, the preferred form will have the full definition, and the runner-up will be cross-referenced to the winner. Online, the spelling in the Web page’s heading indicates the preference, though the other choice will probably also be listed. Here are some common entries with more than one orthography:
1. Acknowledgment/acknowledgement: Acknowledgment, though it looks awkward because the spelling implies that the g is pronounced hard, rather than (correctly) soft, is the preferred spelling, at least in American English.
2. Adviser/advisor: Adviser is the preferred spelling, though it is inconsistent with the spelling of the adjectival form advisory.
3. Aesthetic/esthetic: Aesthetic is the preferred spelling, a rare case of the digraph retained in American English in favor of a single-vowel spelling. (See also amoeba/ameba and archaeology/archeology.)
4. Ameba/amoeba: Amoeba is the preferred spelling. It also has variant plural forms: Amoebas is acceptable in all but the most strictly scientific contexts, where amoebae is preferred.