When giving suggestions to a recent translation question I could not decide between similis and par. I realized that I don't understand the difference of the two sufficiently well. They are obviously distinct but similar words, but dictionaries are not very good for comparing almost synonyms. How would you compare similis and par? How are they similar and how are they different? Is there a useful rule of thumb for differentiating them?
Each is an adjective but, like their antonyms dissimilis and impar, they are neither equivalent nor interchangeable.
Indicating only 'likeness', similis is 'similar' or 'resembling', while par means 'equal' (and does not imply similarity) or 'the same' in some quality (whether stated or implicit) of the noun it qualifies.
For example, simili intervallo is said of objects spaced at similar — but not precisely equal — distances apart, while pari intervallo implies that the intervals are exactly equal. Or you might say when comparing cucumbers and apples cucumeres et mala sunt pondere paria, sed forma dissimilia, and so on.