Hello. Today, I am wondering about use of the preposition ab/ā with nouns being described as being the means of achieving some end. In Latin, the "ablative of means" conveys the notion of such instrumentality, meaning that the instrumental "noun of means" will take the ablative case within the sentence. The ablative of means, though it falls under the primary heading of "instrumental ablative", is generally used, as is the "true ablative" (or "ablative proper"), and unlike most of the other types of instrumental ablatives, without an accompanying preposition, which in the case of the ablative of means, would be ab/ā. A sample sentence might be: ratiōne meā, exercitus citō veniat. (By my reckoning, the army should soon come.") The essence of my question involves whether this was generally done only by convention, out of the knowledge that the preposition in this case was not strictly necessary, or whether there was some rationale which made it a "hard and fast" rule. In short: was this merely done by convention, or was it done pursuant to a grammatical rule. To state my question otherwise: might a writer opt to use the preposition in such a construction, perhaps he is a poet who wishes to do so in order to maintain metre, as in the hypothetical sentence: ā ratiōne meā, exercitus citō veniat, or would so doing be construed as absolutely incorrect? If so, can you cite any examples from the corpus. If not, then why?