When a message is replied to in e-mail, Internet forums, or Usenet, the original can often be included, or "quoted", in a variety of different posting styles.
The main options are interleaved posting (also called inline replying, in which the different parts of the reply follow the relevant parts of the original post), bottom-posting (in which the reply follows the quote) or top-posting (in which the reply precedes the quoted original message). For each of those options, there is also the issue of whether trimming of the original text is allowed, required, or preferred.
For a long time the traditional style was to post the answer below as much of the quoted original as was necessary to understand the reply (bottom or inline). Many years later, when email became widespread in business communication, it became a widespread practice to reply above the entire original and leave it (supposedly untouched) below the reply.
While each online community differs on which styles are appropriate or acceptable, within some communities the use of the "wrong" method risks being seen as a breach of netiquette, and can provoke vehement response from community regulars.
Quoting previous messages
In an e-mail reply, it is sometimes appropriate to include a full or partial copy of the original message that is being replied to; due to the asynchronous nature of Internet communication, people often engage in many conversations at the same time, and email responses may be received long after the original message was sent. For these reasons, the original poster may not be aware of what message a post is intended to be a response to, and providing context is helpful. Many email reading programs (mail user agents) encourage this behaviour by automatically including a copy of the original message in the reply editing window.
Quoted text from previous messages is usually distinguished in some way from the new (reply) text. Often, the two parts are given different indentation. In the example below, the first line is the original message, the second line is the reply:
I can't believe you would say something like this. All these words are terrible. They're hurtful and they make no sense. -- DocEvil
Much like your posting. -- Jim
Alternatively, special delimiter lines may be used:
Hey Joe, Paris is in France, not England. --Mary
--- original message ---
You just had a call from England, from Paris I think. --Joe
--- end of original message ---
For extra clarity, blank lines may also be inserted between the two parts. When using an email medium that supports text markup (such as HTML or RTF), the previous text may be indicated by a distinctive font and/or color:
The meeting has been postponed to next Friday. --Mary
Has the deadline for the report been moved too? --Joe
Quoted line prefix
A common convention in plain-text email is to prefix each line of the quoted text with a distinctive character or string. As of 2020 (and for many years previously), the greater-than sign (">", the canonical prefix) is almost universally used; but other characters such as the ASCII vertical bar character ("|") have been used as well, sometimes with one or more spaces inserted before or after the quoted text marker.
There is no standard declaring one quote-prefix to be "right" and others to be "wrong", but some standards depend on conventional quoting. The "never issued" and obsolete "son-of-1036" draft RFC 1849 recommends ">" as the quote-prefix; RFC 3676 depends on it and considers ">> " and "> > " to be semantically different. That is, ">> " has a quote-depth of two, while "> > " has a quote-depth of one, quoting a line starting with ">". Most e-mail clients treat the two sequences as equivalent, however.
The convention of quoting was common in Usenet newsgroups by 1990, and is supported by many popular email interfaces, either by default or as a user-settable option. In Microsoft Outlook, for instance, this behavior is controlled by an option labeled "Prefix each line of the original". Besides inserting markers automatically in quoted lines, some interfaces assume that a line starting with a ">" character or similar is quoted text, and will automatically display it in a distinctive font or color:
> How is the report coming? --Mary
It will be on your desk by noon. --Joe
Sometimes the insertion of a quoted line marker will cause one original line to be folded as two lines in the reply, and the continuation line may not have the proper marker. To avoid ambiguity in such cases, one may consider inserting blank lines after each block of quoted text: