Vines

Vines are non-solid blocks, $1/16$ of a block in thickness, that can only be placed against a solid block onto walls. They also can be supported by a solid block as a ceiling.

Obtaining
Vines can be destroyed with anything, but mining them using shears is the only way to collect them. Efficiency on shears does not work for vines. The mining speed can be increased with an axe, but an axe cannot collect vines. If multiple vines exist within the same block space, breaking one of them will break all of them instead of just the one targeted and will result in only a single vine dropping ; despite their inconsistent nature, these are both completely intentional features and not bugs.

Natural generation
Vines are naturally generated in jungles and swamps. Jungle trees of both sizes will have vines on their trunks and canopy edges. Vines are also naturally generated in oak trees in swamplands. Vines are generated naturally in jungle temples and woodland mansions. Vines can also generate on dying trees of any kind.

Growth
On each block tick, the vine block has a 25% chance of attempting to spread in a randomly-selected direction. Vines will spread to adjacent blocks under certain conditions:  Downwards:  If the block below is air, each side (north, south, east, and west) with vines has a 50% chance to spread downwards. If the block below is vines, each side of that block with vines on the ticked block has a 50% chance of gaining vines.  Upwards:  If the block above is air and there are fewer than 4 other vine blocks in a 9×9×3 area surrounding the ticked vine block, each side (north, south, east, and west) with vines has a 50% chance to spread upwards if that side is backed by a block that vines can be placed on.  Horizontally:   If the current vine block isn't covering the target direction and there are fewer than 4 other vine blocks in a 9×9×3 area surrounding the ticked vine block, then:  If the block in the target direction is air, then it will try:   If the vine block has vines to the right and that side of the air block is backed by something vines can be placed on, spread to the right side of the air block.</li> <li style="clear:right"> If the vine block has vines to the left and that side of the air block is backed by something vines can be placed on, spread to the left side of the air block.</li> <li style="clear:right"> If the vine block has vines to the right, and that side of the air block is backed by another air block, and the right side of the vine block is backed by something vines can be placed on, spread around the corner.</li> <li style="clear:right"> If the vine block has vines to the left, and that side of the air block is backed by another air block, and the left side of the vine block is backed by something vines can be placed on, spread around the corner.</li> <li style="clear:right">If the air block in the target direction has something vines could be placed on above it, spread to the top side of the air block.</li> </ul></li> If the block in the target direction is opaque and fills its whole cube, spread to that side of the ticked vine block.</li> </ul> </ul></li></li> </ul>

Usage
Vines can be placed on the side of any block that fills its whole cube and blocks entity movement. They cannot be placed on the bottom side of a block, despite being able to spread there on their own, and a vine block without vines on any of the horizontal sides will disappear if it gets an update from a neighboring block. Vines attached to the bottom of stairs will be completely deleted by any block update.

Several vines can be placed in the same block space, as long as they are facing different directions.

Vines block mobs' line of sight, so it is possible to sneak up onto mobs by hiding behind vines. Because of this, endermen can safely be viewed through vines and skeletons will not attempt to shoot targets behind vines.

Vines can be climbed, but only when a solid surface can be pressed against while standing in the vine block. Generally this means a solid block the vines are growing on, but it is possible to climb hanging vines if there is a nearby barrier of some sort.

If the player is sprinting and runs through hanging vines, they will slow their movement speed or cancel the sprint.

Vines absorb all fall damage, even without a solid surface nearby. on vines causes the player to hang, even if the vines are not adjacent to any surface.

Blocks can replace vines if placed within the same block space. However, item frames and paintings cannot.

Block data
Determines the face against which the vine is anchored. Note that (except for Top) these are testable as bit flags, unlike most of the other directional data for other block types. Multiple sides can contain vines. The "top" attachment is assumed to be present if data is 0 or there is solid block above.
 * 1:
 * 2:
 * 4:
 * 8:

Video
''Note: This video is outdated. It states that vines are not used in any crafting recipes, which is not true - they are used in crafting Bordure Indented Banners, as well as Moss Stone and Mossy Stone Bricks.''

History
When the Adventure Update was showcased at PAX, vines hadn't been given a name, and a mouseover would not show a title. This implies that they didn't have an official name at the time.