Ladder

Ladders are wooden blocks used for climbing walls either vertically or horizontally. They can only be placed on the sides of other blocks.

Placement
Like torches, a ladder occupies one block from the side on which it is placed and cannot be placed on glass. Unlike torches, however, they resist water and will create an air pocket. They also resist lava, and can be used instead of glass to let lava light shine through a ceiling. If you place a ladder in lava or water just on the ground not near a wall, it will still have an air pocket but will be invisible, still there though, thus the invisible block.

Ladders are classified as a solid block, although this may not be apparent due to their thin nature. It is possible for a player to stand on the narrow top surface of the ladder, and damage can be incurred by a mob if it falls on the thin strip. Spiders can be blocked from climbing a wall by placing a ladder at the top edge of the wall, so they get stuck on the bottom edge of the ladder.

Usage
Ladders provide a way to safely cushion a player's impact after a free fall. Upon entering a ladder's area of effect, the player's speed is reduced instantly to normal ladder descent speed and no damage is taken. Once again, care must be taken to avoid landing on the narrow top surface of the ladder.

Since Beta 1.4, holding the sneak key whilst climbing a ladder will grab hold of the ladder, to prevent you from falling off. However, when mining a block, the rate which a block is mined decreases while the player is holding on to a ladder. This effectively allows to break the "never dig straight down" rule.

Any mob can climb a ladder in the same way a player does: by pushing against it. Mobs are not smart enough to deliberately use ladders — they won't stay on a ladder to get somewhere — but a ladder directly in their straight-line path will let them climb up.

How ladders work
A ladder takes effect whenever your lower half is in the cube occupied by a ladder. These effects are:
 * If you push against a wall, you move upward instead. (Thus inside a 1×1 ladder shaft, you can use any movement key to move upward.)
 * Your maximum downward speed is reduced to a “descending ladder” speed.
 * Your horizontal movement friction is the same as on ice. (That is, moving around in the tiny space you can be in while still hanging on the ladder at the same level.)

History
Ladders were added in Infdev (June 7, 2010).

Pressing the crouch key to stop moving down the ladder was added in Beta 1.4.

Before Beta 1.5 ladders could be placed every other block and still be climbable, because you could climb a ladder when any part of your body was on it, not just the lowest part. As well, the ladder crafting recipe produced only one ladder per 7 sticks instead of two; thus the 1.5 changes did not change the number of sticks needed to make a ladder of a given height.

Trivia

 * It takes approximately 52 seconds to travel from bedrock to the roof of the sky on ladders.
 * Placing ladders on snow layers make the ladder invisible but they still act as if you placed them normally.
 * If you place a bucket of water in a place occupied by a ladder with no other blocks next to it, the water will replace the ladder and you will not get the ladder back.
 * You can use ladders with having a space between them. Placing the ladders in a corner and then shift between the two walls. This method is a bit hard at use as you need to jump and shift side constantly. But it works!
 * By holding the jump key, players can still use water ladders that have ladders placed every other block.
 * If you stop yourself on a ladder and place a sign, you will still stay in midair without having to hold Shift.
 * Attempting to place a ladder on a spot another ladder occupies may instead place the ladder somewhere near by.
 * As of Version 1.5_01, it is possible to remove a ladder while standing on a ladder and holding the sneak key, but it can take a long time because the countdown for breaking the ladder often resets.