Top-posting

Top-posting occurs when someone replies to your message by adding reply content to the top of the message that you sent to them, usually leaving your whole message intact at the bottom:

    From: manager
    To: you

    Seventeen.

    >From: you
    >To: manager
    >
    >How many should we buy?
    >-- 
    >Acme Rubber Band Works
    -- 
    Acme Rubber Band Works

The added content precedes the message to which it applies, so it appears out of order. A message forwarded back and forth with added top-posting has to be read backwards to be understood, from bottom-to-top, since the newer material is added on the top rather than on the bottom:

    From: you
    To: manager

    Is that metric or imperial?

    >From: manager
    >To: you
    >
    >Seventeen.
    >
    >>From: you
    >>To: manager
    >>
    >>How many should we buy?
    >>-- 
    >>Acme Rubber Band Works
    >-- 
    >Acme Rubber Band Works
    -- 
    Acme Rubber Band Works

In top-posting, little or no editing is done on the quoted reply. Every message is quoted and re-quoted in its entirety - address, signatures and all. People who top-post and only read the first screen of the message are oblivious to the size of the text they are quoting; dozens or hundreds of lines of quoted material get re-quoted and passed on in every message. The message gets larger and larger. The redundant signatures pile up at the bottom of the message.

Bottom-posting

Bottom-posting occurs when someone replies to your message by adding content to the bottom of the message you sent, after trimming your original message down to remove signatures and irrelevant quoted context:

    From: manager
    To: you

    >From: you
    >How many should we buy?

    Seventeen.
    -- 
    Acme Rubber Band Works

A message forwarded back and forth with added bottom-posting reads in a logical order from top to bottom, like a story:

    From: you
    To: manager

    >From: manager
    >>From: you
    >>How many should we buy?
    >
    >Seventeen.

    Is that metric or imperial?
    -- 
    Acme Rubber Band Works

The content is in logical, top-to-bottom order. Extra signatures and irrelevant context have been removed. The messages are shorter (often much shorter) and easier to read.

Top-posting forces your reader to work

A top-posted message is easy to write; since, it doesn't require the writer to edit any of the original message. As a consequence, a top-posted message makes the message reader do the work of understanding what was said. Anyone new to a top-posted conversation has to read a message unnaturally from bottom to top. A reader may have to skip over the new top-posted comment at the top of the message and scroll down to the bottom of the message to see the context for the comment, then scroll back up, message by message, to read the added comments in an unnatural bottom-to-top reverse order. No editing has been done by the top-posting writers; so, each message contains redundant signatures and irrelevant material.

Anyone reading a message digest, where multiple messages are saved up, joined together, and delivered as one big message at the end of the day, will find all the quoted material appearing over and over in the digest. Since the quoted material is usually much larger than the new material added, skipping over the quoted material in the digest often causes new material to be accidentally skipped as well. Top-posted email digests are huge and unreadable.

People who archive their incoming email and search it for keywords also find top-posted email awkward. Instead of finding one or two messages, a keyword search finds all the messages, since all the messages contain quoted copies of all the other messages.

A message passing through a series of top-postings becomes harder and harder to read; since, the information in the story line is presented in an unnatural backwards order, from bottom to top. The important content is surrounded by unnecessary signatures and irrelevant context.

Some argue that you should already know the context for a top-posted reply; so, scrolling down to read the quoted context should be unnecessary. If you expect your reader to remember the context, you don't need to quote the original message. If you think the quoted reply is necessary, edit it down and bottom-post under it to make it read in a logical order.

Remembering context is also not true for people who deal with any significant volume of email. Your email may be only one of dozens (or hundreds) of simliar emails; don't expect your reader to remember the context of your top-posted reply. Edit your quoted material and bottom-post under it.

Knowing the context is also not true for people reading archived discussions, where the people reading the messages are either away from the discussion for a long time or are not the ones who originated the messages. A top-posted message thread forwarded to a third party also subjects the third party to a lot of unnecessary work reading bottom-to-top.

The signal you are sending along with a top-posted reply is:

A new reader of a top-posted message has to decipher the correct order of the unnaturally reversed parts of the message. Multiply this work by dozens or hundreds of readers, and a top-posted message makes a lot of other people do a lot of unnecessary work.

Bottom-posting is easier on your reader

A bottom-posted message makes the message writer do the work of preparing the message for easy consumption by the reader. The writer has work to do - to edit the original message to remove signatures and irrelevant context and then add the new comment to the bottom of the message.

The reward is that a message passing through a series of bottom-postings reads in a logical order from top to bottom. Bottom-posting (with editing out of irrelevant context) keeps a message short and easy to read and understand.

The signal you are sending along with a bottom-posted reply is:

Since the writer of a bottom-posted message has done the work to make the message easy to read, the dozens or hundreds of people reading the message don't waste time reconstructing it. A lot of human time is saved. (A lot of mailbox space is also saved!)

Better bottom-quoting for MS Outlook users

Outlook-QuoteFix can modify MS Outlook's message composition windows on-the-fly to allow for correct quoting and to change the appearance of your plain-text replies and forwards in general: move your signature, use compressed indentation, customize your quote header, etc. Try it:
http://home.in.tum.de/~jain/software/outlook-quotefix/

Arguments to support bottom-posting...

    A: Because we read from top to bottom, left to right.
    Q: Why should I start my reply below the quoted text?

    A: Because it messes up the order in which people normally read text.
    Q: Why is top-posting such a bad thing?   

    A: The lost context.
    Q: What makes top-posted replies harder to read than bottom-posted?

    A: Yes.
    Q: Should I trim down the quoted part of an email to which I'm replying?
http://mailformat.dan.info/quoting/bottom-posting.html
http://www.xs4all.nl/~sbpoley/toppost.htm
http://www.dickgaughan.co.uk/usenet/guide/faq08-topp.html
http://www.xs4all.nl/%7ewijnands/nnq/nquote.html
http://www.cs.tut.fi/~jkorpela/usenet/brox.html
http://www.caliburn.nl/topposting.html
http://www.mail-archive.com/brin-l@coollist.com/msg00178.html

Wikipedia

Top-posting
Top-posting discussion

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