That is a pair of glasses -- grammatically correct and normal. Those are a pair of glasses -- might be said by a native speaker influenced by the notional plurality of glasses.
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Mister MicawberYes. Those are glasses is OK and That is a pair is OK.In comparison, however,
Or: Those are scissors / That is a pair of scissors. These are pants / This is a pair of pants.
Mister MicawberIf I understand this correctly, grammatically correct sentences should be
Glasses, scissors and pants are called 'summation plurals'. Number contrast is achieved by means of 'a pair of'. Thus 'a pair of glasses' refers to one item, while 'two pairs of glasses' refers to two items. ( I paraphrase Quirk, CGEL.)
Mister MicawberYes, but you can also say 'that is a pair of tigers'. It depends on whether you are focussed on one pair or two tigers.So to summarize,
Mister Micawberyou can have one tiger but not one glass – that is the only real point to be taken from this.No, the real point of my question was wheather "a pair of (plural noun)" should be treated as singular or plural.