Sign

Signs are blocks that have text on one side of them. Signs may be used to mark property, leave directions, state rules, or even leave out your poetic stylings for travelers to stumble upon. Signs can also be used to create furniture such as the sides of chairs or ends of beds.

Signs are one of the many blocks that are broken more quickly with a sword, but still deal double damage to the sword. The breaking animation also doesn't play.

History
Signs were originally added in Survival Mode, but removed shortly after. They were re-introduced in infdev, where they would have greater use. As of the 07/02/10 update (Seecret Friday update #3) signs became smaller.

As a block
Signs count as a regular block, except mobs and items can pass through them. This means that other blocks can be placed on top of a sign, and wall-clinging blocks can also be placed on their sides. This also means that mobs will recognize them as solid and try to walk over them even if the signs are placed over high drops, which makes convincing mobs to fall down high drops (such as in mob traps) convenient. With the 12w04a snapshot, however, they seem to recognize signs as non-solid, and will avoid walking over them if the drop below is too tall.

Any kind of liquid cannot enter the space occupied by a sign, so signs, like ladders, may be used to make a doorway through which lava or water cannot flow.

Writing
After placing a sign, a window pops up where the player may input text to be placed on the sign, although signs may also be left blank. Four lines of text are available, with 15 characters on each line. To switch between lines, the player can press Enter or the up/down arrow keys. The line which is currently selected will have a greater-than and less-than sign (> and <) at the beginning and end of it. While editing, within a single line text can only be entered and deleted in an ordinal manner, meaning that it is impossible to insert text in the middle of an already typed line - the player must delete and reenter text in order to make edits. Once a line has 15 characters, no more text may be entered on that line.

After the editing window is closed, the sign's text may only be changed by removing it (which will discard the text) and placing it again. In singleplayer, the game pauses until the player finishes typing.

Placing
One can place signs in the ground sticking out, or on walls without any post grounding them. Signs may be placed attached to any block, including ones such as fences, glass, other signs, and minecart tracks, which torches cannot be attached to all except to fences in 1.7.2 and above.

Free-standing signs can face one of 16 directions, and face toward the player when placed. Wall-mounted signs can only face the four cardinal directions, and are attached to the face the player clicks on of an existing block.

Before Beta 1.6, it was possible to create a Water Ladder due to the South/West rule, which allowed the player to ascend quicker.

If a sign is placed underwater, either on the side of a block or on top of another sign, an air bubble will form, allowing the player to breathe underwater.

Colored text
Text in signs can be colored, but only with a Bukkit plugin. The section sign (§), or the ampersand (&) followed by a number from 0-9 or a letter from A-F, will change the color of the following text on the current line. The section sign can not be entered via Minecraft itself; a map editor is required to edit the text to enter the § character. However, the ampersand (&) can be entered by holding Shift and pressing 7 (or 6, depends on keyboard) on most keyboards. The section sign and the following hexadecimal number also count towards the 15 character limit. Note that the ampersand may not work on all computers.

Bugs

 * In SMP, if a player destroys a placed sign while another player is writing on it, the text prompt will not disappear.
 * Like Paintings, Signs do not use the Smooth Lighting function when enabled.
 * If a player enters all fifteen characters on a line as tildes (~), the first and last two tildes appear in mid-air, instead of on the sign. This also occurs with a line of 15 "at" symbols (@).
 * The grave character ( ` ) cannot be typed on a sign.
 * If a sign is placed during the portal animation, "w" will appear on the first line of text, and no text can be entered.
 * A sign can be placed before an already placed sign.
 * In SMP, when players do not have permission to remove a sign, they can still delete the text by destroying it, after which the sign will come back, but blank. This is client-side, however, and the text reappears after disconnecting and reconnecting.
 * In SMP, if a player places a sign and starts writing text,but does not finish, another player can edit the text on the sign if he does not have permission to destroy the sign, but destroys it anyway,and places another sign in place of the original.(The second player must have finished writing the text before the original for the changes to take place.)
 * In SMP, if a player puts a sign on a piston, the game will crash, but the server will continue to work.
 * If you hover over a sign in Creative mode, the sign will make wooded foot noises, even though you can't stand on the sign.
 * In SMP, if a player places sign near a pulsing repeator, text will vanish every half a second (in editing window).
 * In SMP, if you put a torch (not tried with others) on the block the sign is placed, and a player is writing, the text the player was writing and still writing will clean.

Trivia

 * You can attach a sign on a sign.
 * A sign attached to a fence seems to float in the air.
 * Once you're done editing a sign, you are unable to edit it again.

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