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1. Since two different protocols, and possibly two different servers, are used to send and receive mail, it is possible that mail clients can perform one task and not the other. (Is the subject for 'are' 'protocols' only? It is because we have commas in there?)

Thanks.
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Hi Jack,

1. Since two different protocols, and possibly two different servers, are used to send and receive mail, it is possible that mail clients can perform one task and not the other. (Is the subject for 'are' 'protocols' only? It is because we have commas in there?)

Let's change the example to one with singular words. Since Tom, and possibly Mary, are going to be late, we will start the meeting without them. It seems to me that the subject is plural. 'Is' would not work.

Thus, the subject in the original sentence includes both the protocols and the servers.

Best wishes, Clive
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The are grammatically agrees with protocols. I believe that the Columbia Guide to Standard AmE addresses this problem as a problem of the 'near-compound' subject, because (to me) the commas, creating 'non-restrictive' content, are quite similar to m-dashes in their intent:

'In compound like constructions with with, together with, and the like, and those in which the second element is set off with dashes or parentheses (The husband, with his wife and baby, comes in every morning; Fred—and Mary when she has time—jogs every morning), usually, if the first element of a near-compound subject is singular, the verb will be singular, and if plural, the verb will be plural, as in The parents—and their large dog—are waiting for the school bus. But because of the so-called principle of proximity, no matter how singular the first element, enough intervening plural material will cause the verb to be plural too, unless the sentence is carefully edited. Hence most problematic instances occur at the Conversational levels of speech and Informal writing, not at Oratorical levels or in Edited English.'
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Mister MicawberThe are grammatically agrees with protocols.  I believe that the Columbia Guide to Standard AmE addresses this problem as a problem of the 'near-compound' subject, because (to me) the commas, creating 'non-restrictive' content, are quite similar to m-dashes in their intent:

'In compound like constructions with with, together with, and the like, and those in which the second element is set off with dashes or parentheses (The husband, with his wife and baby, comes in every morning; Fred—and Mary when she has time—jogs every morning), usually, if the first element of a near-compound subject is singular, the verb will be singular, and if plural, the verb will be plural, as in The parents—and their large dog—are waiting for the school bus. But because of the so-called principle of proximity, no matter how singular the first element, enough intervening plural material will cause the verb to be plural too, unless the sentence is carefully edited. Hence most problematic instances occur at the Conversational levels of speech and Informal writing, not at Oratorical levels or in Edited English.'

May I ask where you got that info from? I would like to use that as a reference for future references.
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