DD is an HTML tag. It is meant to be the description part in a description list. It should contain information that explains or clarifies or defines what was contained in the DT tag that should precede it.

The spec

DD appears in HTML 2.0. It is one of those tags that improve with age, like a fine red wine. It is defined as

<!ELEMENT DD - O %flow>

... a charming SGML chunk meaning that the tag need not be closed, and that it contains %flow. What is %flow ? It is defined as (%text|%block)*. This means block level elements and text level elements, that's to say almost anything you can think of.
You can nest a DL list inside a DD element.

At this point we have only discussed the content of a DD element. But where can it appear ?
The spec is quite specific (res ipsa loquitur):

<!ELEMENT DL - - (DT | DD)+>

... and this is the only place where DD appears in the HTML 2.0 spec. And this means, people,

You can put a DD only inside a DL element. Not on its own. Never.

or rather, you can put it anywhere, but that is not legal HTML. Why should you care ? Accessibility, portability, being nice.

More spec

Not much to say here. The definition does not change in HTML 3.2 and HTML 4.0. The above big dictum still holds.
I see that you are so hip that you want to know about XHTML. Things will not change much, but as you well know you will be required to close the DD tag with a corresponding </DD>.
After scuba diving into the W3C site and poking at the XHTML 1.1 DTD, I come up with the same damned fact about the DD element being legal only inside DLs. And of course, since XHTML is XML, all tags need to be closed, so you need to provide </dd> tags. Notice also that in XHTML all markup must be in lowercase, so the tag is in fact dd, and not DD.

But I wanted to indent

Use the BLOCKQUOTE element. Use wisely the &nbsp; for things like poetry. Outside of Everything2 you should be using CSS anyway: it allows fine control, and cleanly puts presentation where it belongs.

You sound like a very repressive, disturbed person

Yes. I like standards. I side with the W3C. The spec is right, the world is wrong. You do want portable content, don't you ? You don't want your WUs to be quoted in a FAQ as "the most fucked up example of markup this side of BLINK", do you ? You are noding for the ages, right ?
So, read the E2 HTML Tags, read the Everything University, look inside your heart and only use the DD tag inside a DL list.

day mode = D = DDT

dd /dee-dee/ vt.

[Unix: from IBM JCL] Equivalent to cat or BLT. Originally the name of a Unix copy command with special options suitable for block-oriented devices; it was often used in heavy-handed system maintenance, as in "Let's dd the root partition onto a tape, then use the boot PROM to load it back on to a new disk". The Unix dd(1) was designed with a weird, distinctly non-Unixy keyword option syntax reminiscent of IBM System/360 JCL (which had an elaborate DD `Dataset Definition' specification for I/O devices); though the command filled a need, the interface design was clearly a prank. The jargon usage is now very rare outside Unix sites and now nearly obsolete even there, as dd(1) has been deprecated for a long time (though it has no exact replacement). The term has been displaced by BLT or simple English `copy'.

--The Jargon File version 4.3.1, ed. ESR, autonoded by rescdsk.

Despite being deprecated, dd has an incredible number of uses; I honestly have no idea how I'd live without it. As the Jargon File entry says, is has no replacement.

The simplest function dd is capable of is copying one file to another.

dd if=infile of=outfile
dd < infile > outfile
because stdin and stdout are the default in and out files.

That's all well and good, but cat can handle that. Where cat is occasionally handy, though, dd is industrial strength. For example, let's extract the boot sector from a floppy disk:

dd if=/dev/fd0 of=/tmp/bootsect.img bs=512 count=1
copies the first 512 bytes of the disk to a file.

dd if=/dev/fd0 of=/tmp/bootsect.img bs=1 count=512
does it as well, but the former is one 512-byte read, whereas the latter is 512 1-byte reads. Who cares? ariels points out that some devices have native block sizes, such that any read will force at least a block to be read. For example, reads from most disk drives occur in 512 byte sectors; thus one should do reads in multiples of 512 bytes for best speed. Note that we restrict count, the number of blocks of size bs transferred, in each case; we only want the first 512 bytes. The default behavior is to go until EOF on the input file.

Now things are getting interesting. This is just the beginning, though -- dd has even more strange capabilities. Adding a skip=n option causes dd to skip the first n bytes of the input file; adding a seek=n option causes it to seek n bytes into the output file before beginning its copy. This is great for resuming a copy where it left off, or copying out a chunk of a file (for example, extracting a file in the body of an email from the headers), or piecing together a file from debris (such as lost clusters collected in /lost+found).

But wait, there's more!

dd also has a conv option, which causes it to convert between types of files. It can convert between ASCII and EBCDIC, convert files between sequential and random access (that is, between fixed-length records and newline-terminated variable length records, AKA lines), convert between lower and upper case, and swap bytes to correct the endianness of a file.

My favorite conversion, though, isn't really a conversion at all -- it's the noerror "conversion". This causes dd to ignore read and write errors and proceed anyway. I have saved numerous floppy disks, rejected by Windows, with the following command:

dd if=/dev/fd0 of=/tmp/floppy.img bs=1 conv=noerror
followed by a swap of disks and a dd in the other direction. Not perfect, but it's a real lifesaver. The files this doesn't recover can be extracted with another dd -- use less to find them, then dd them out of the image.

A word to the wise about this method: Windows does not like the disks thus created. The solution is to mount the disk in UNIX, move the files off it, reformat it, and move them back on. UNIX can handle the blank spaces left where dd skipped bad sectors; Windows barfs or freezes upon reading them. (Score one for UNIX, and another for dd!)

It should be mentioned at least in passing that DD is also a bra cup size.

Seems that manufacturers started out with 4 standard sizes: A, B, C, D (going from small to big, or from Kate Moss to Geri Halliwell. But then there are always a few specimens that don't fit into this range, yet clamoring for a bra size of their own. So they gave Brigitte Nielsen size AA and Pamela Anderson (thanks to a generous implant of silicone) DD.

Size DD is eminently impractical. DD breasts keep getting in the way. They're a concussion hazard when in motion and a smothering risk when you get too close. However, I must say they're nice to look at from a respectful distance.

Breast Files, http://www.breastfiles.com/bras.html

Abbreviation for "duplex drive", a modification to tanks to make them amphibious, developed by the British military inventor Major General Percy Hobart during the Second World War.

The eponymous drive was basically just a pair of propellers linked up with the transmission which could drive the vehicle through the water at a speed of around four knots. The more visually striking part of the system, however, was the pneumatically-erected canvas flotation skirts. These rose from a frame mounted around the hull of the tank to rise a couple of metres above the top of the turret; the tank itself floated below the waterline, so that all was visible was what looked like a shallow canvas boat. The tank was steered by the driver altering the relative speed of the two propellers (not dissimilar to the way you steer with tracks); the tank commander could just see out to navigate by standing on top of the turret. The crew were issued with primitive breathing apparatus in order to try and escape if the vehicle was sunk.

The system was initially tested on Valentine and Tetrarch tanks, but eventually saw action as a modification to the ubiquitous M4 Sherman. In the preparations for the D-Day landings DD tanks were allocated to the British army's 79th Armoured Division, a specialist amphibious assault and engineering formation which also operated many other vehicles of Hobart's invention, to some units of the Canadian army, and (alone) of Hobart's "funnies", to the US army. On D-Day itself several regiments of DD tanks were launched from 4 km off shore and fought with considerable success on four out of the five landing beaches, but most of the tanks in the US 741st Armored Battalion allocated to Omaha beach were swamped after having navigational problems which led to their getting beam-on to the fairly rough seas.

Although D-day used up the element of surprise for the design, DD tanks were subsequently used in various river crossings, including the attack across the Waal to capture Nijmegen bridge during the Arnhem operation.

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