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When I reply a party invitation, I often use the following:

1) John, David, and I will be there.
2) John, David and I will be there.

I saw both very often. Which one is right? Please help.

In William Strunk, Jr. (1869–1946). The Elements of Style. 1918.
In a series of three or more terms with a single conjunction, use a comma after each term except the last.

Thus write,
red, white, and blue honest, energetic, but headstrong He opened the letter, read it, and made a note of its contents.
This is also the usage of the Government Printing Office and of the Oxford University Press.

Comments  
I personally would use the two commas ...
Both are correct. American English favours the comma; I'm not sure if British English has a preference.

There are sentences, where the comma before the "and" is essential, even if you favour leaving it out.

Listing book-titles: Jane Eyre, Romeo and Juliet, and Oliver Twist. (That would be an interesting book: "Romeo and Juliet and Oliver Twist" hehe...)

Then, there are cases, where leaving out the comma before the and causes ambiguity. I like the example given by the Oxford Dictionary of English:

Wrong: "I would like to thank my parents, Anne Smith and God." (it's: "I would like to thank my parents, Anne Smith, and God." - otherwise Anne Smith and God would be your parents.)

So, if you're already leaving out the comma before the "and" by default, there's no reason to change that (but be aware of ambiguities). If you're just learning English, though, it may be easier to make the comma before the and, as you're less likely to produce confusing sentences.
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DawnstormBoth are correct. American English favours the comma; I'm not sure if British English has a preference.

There are sentences, where the comma before the "and" is essential, even if you favour leaving it out.

Listing book-titles: Jane Eyre, Romeo and Juliet, and Oliver Twist. (That would be an interesting book: "Romeo and Juliet and Oliver Twist" hehe...)

Then, there are cases, where leaving out the comma before the and causes ambiguity. I like the example given by the Oxford Dictionary of English:

Wrong: "I would like to thank my parents, Anne Smith and God." (it's: "I would like to thank my parents, Anne Smith, and God." - otherwise Anne Smith and God would be your parents.)

So, if you're already leaving out the comma before the "and" by default, there's no reason to change that (but be aware of ambiguities). If you're just learning English, though, it may be easier to make the comma before the and, as you're less likely to produce confusing sentences.
I'm not sure how wide-spread it still is, but the comma before the 'and' is officially known as the Oxford comma (I suppose the reason is obvious). Reminds one of Lynne Truss' delightful Eats Shoots and Leaves, reading I would recommend on both sides of the Atlantic.
(1) John, David, and I will be there.
(2) John, David and I will be there.

I have been taught that in AmE, the comma is used before 'and' as in the example above.

In BrE, no comma is needed. And this is also what English usage books say.
PhilipI'm not sure how wide-spread it still is, but the comma before the 'and' is officially known as the Oxford comma (I suppose the reason is obvious). Reminds one of Lynne Truss' delightful Eats Shoots and Leaves, reading I would recommend on both sides of the Atlantic.
Typing "Oxford Comma" into google yields interesting articles. Thanks.

Is the book called "Eats Shoots and Leaves" or "Eats, Shoots and Leaves"? Emotion: wink
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Oxford comma
It's also known as the "serial comma".

In ordinary BrE usage (e.g. company documentation, government publications, newspapers), you rarely see the Oxford comma, unless there's a possibility of confusion.

It may turn up in publications from some university presses, however.

The same is true of -ise/-ize endings in BrE: -ise is favoured in ordinary usage, but -ize is preferred by some publishing houses, except in cases such as "advertise", "improvise", etc.

Both the Oxford comma and -ize endings were predominant in BrE publications till c.1950.

MrP
Dawnstorm
PhilipI'm not sure how wide-spread it still is, but the comma before the 'and' is officially known as the Oxford comma (I suppose the reason is obvious). Reminds one of Lynne Truss' delightful Eats Shoots and Leaves, reading I would recommend on both sides of the Atlantic.
Typing "Oxford Comma" into google yields interesting articles. Thanks.

Is the book called "Eats Shoots and Leaves" or "Eats, Shoots and Leaves"?Emotion: wink
Without the comma: the clever cover illustration has one panda walking away with a pistol in hand and another panda on a ladder whiting out the comma after "eats".
Sorry Philip, but the book is with the comma. Without the comma, the title just means that shoots and leaves are what the panda eats.
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