What about the usage of terms such as:
- Historic vs historical
- Geologic vs geological
- Biologic vs biological
Great question, John! Odd pair of suffixes, those, and between them they form a mountain of adjectives that may or may not differ in meaning.
Both suffixes mean “of or relating to” and come from the Latin suffix -icalis, which means the same thing. For example, clericalis means clerical.
The -ic suffix gets the main entry in Merriam-Webster, while -ical gets a shorter one that cross-references the first.
Although often -ic and -ical words formed from the same root are synonymous, writers are advised to use caution, because a lot of those adjective pairs have undergone differentiation. In Garner’s Modern American Usage, Bryan Garner advises us to “keep a couple of good dictionaries nearby to help you decide which adjective to use.”
For example, historic is used for momentous things and people with lasting import, while historical refers to anything that happened in the past. So for example, the election of the country’s first black president was historic, even though it happened recently, and a photo of my great-great-grandmother is historical, even though you won’t find her in a history book.
Other times, one form becomes a noun while the other becomes the sole adjective, as with biologic and biological.
In addition to differentiation, we also need to remember that in some cases there is no distinction, so it’s best to reject what Garner calls “needless variants.”
How do you tell which variant is needless? The preferred term will have a headword entry, and the variant will be listed after it.
ecological also ecologic
geologic or geological
As we’ve seen before, when lexicographers separate the variants with “also,” the first-listed word is the preferred one, whereas when they are separated by “or,” either is acceptable. But usage experts like Garner recommend using the first-listed word in the second case also.