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I didn't see an element with id 1509429 but it still works,how?

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That is referring to an answer rather than a question. The first number in the URL is the question number. The name of the question can change if someone with authority (permission) edits the title, so I suspect that the value in the permalink is immaterial.

Indeed, some playing with permalinks shows that the text portion is effectively ignored; you can reduce it to a single character, or remove all the characters, or remove the slash too. Thus, all these get to this answer:

The first of the two numbers separated by the '#' (hash) symbol is the answer number - as part of the URL, it instructs Stack Overflow to load the page with that answer on it (only really meaningful for questions with multiple pages of answers, but a link wouldn't be very *perm*anent if it broke once too many answers were posted).

The number after the hash (the URL "fragment") is also the answer number - but this time it instructs the browser to scroll that answer into view once the page has loaded.

Oh, and you're right - there's no element with an id set to the answer number; it's an honest-to-goodness anchor!

<a name="1509429"></a>

(Thanks to @shog9 for some previous additional information - the last couple of paragraphs, roughly.)

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Can you explain what <a name="1509429"></a> exactly means?I've never seen the name attribute to work in the DOM other than posting values. – Shore Oct 4 '09 at 8:06
@unknown: see – Shog9 Oct 4 '09 at 8:10
Oh,I've never known about this before! – Shore Oct 4 '09 at 8:20
But this still doesn't explain why 1509429#1509429 is neccesary,which is my third comment to Jeff Atwood. – Shore Oct 4 '09 at 9:55
@Shore - Actually it does - Step 1) The request returns the question with the page selected for that answer (specified by /1509429 - this doesn't happen if you leave this portion of the URL out), Step 2) The browser loads the page, and moves to the anchor specified by #1509429 - no need for any JavaScript to perform this action (which would be required if you left off the hash portion of the URL) – gnarf Oct 4 '09 at 11:33
@shog9 - thanks for the assist. I suspect you should be getting more of the credit than me. :D – Jonathan Leffler Oct 4 '09 at 19:47

hint: one of these things can be seen by the server and the client browser, while the other can only be seen by the client browser.

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I think you used url rewrite,can you tell me what's the url after rewriting? – Shore Oct 4 '09 at 8:11
Is it something like /questions?id=1#3212? – Shore Oct 4 '09 at 8:18
But I don't see the reason to twice the number like /1509429#1509429,isn't /1509429 enough? – Shore Oct 4 '09 at 8:19
#1509429 directs browsers (even with JavaScript disabled) to scroll down to the named anchor point in the document. – gnarf Oct 4 '09 at 11:34
What gnarf said, with the addition that this information is not passed to the server, and so it is necessary to include that tag a second time so the server knows which answer you're talking about. – Joel Coehoorn Oct 4 '09 at 20:05
@ Joel Coehoorn,if I can check a comment as the answer,I'll do it on you! – Shore Oct 6 '09 at 17:50

The old way for bookmarks to work was with a named hyperlink, and this is the method that SOFU uses, for example:

<a name="24523"></a>
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Named anchors (<a> tags) are a way to link to specific parts of a page..

Say you have a long page, and in the middle, the following HTML:

<a name="1509429"></a>

When you navigate to the browser will jump to where the above HTML is.

Simply put, the #1509429 makes the browser scroll the appropriate answer, rather than the top of the page (as a normal link would)

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