The use of 'Sir': Capital 'S' or small?

[Tom]

Hi

I have seen both versions--could you please tell me if both are correct?

[Yankee]

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I'd use your first version, Tom.
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[Alan]

Hi Tom,

The usual form of address would be lower case 's'. The capital 'S' could be used in narrative when perhaps the man was of particular importance as with a woman 'madam' 'Madam'. Then of course in the UK the capital 'S' + the first name of a man is used as a title - a so-called knighthood given by the monarch if the individual has made a major contribution in a particular field. Further details could no doubt be supplied by our friend, Englishuser.

Alan
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[Tom]

Many thanks, Amy and Alan

In Jane Eyre, throughout "Sir" has been written with a capital 'S'--however in my book "Skills in English", at many places it has been written with a lower case.

Tom

PS: Is my use of throughout correct here?