Glass Pane

Glass Panes are transparent blocks which can be used as an alternative to glass blocks. They connect to adjacent panes, much like redstone, iron bars, and fences. They have the same texture as glass. Glass panes are used for the windows of villages. When placed without anything on the sides of it, they will appear in the shape of a from the top, but when something is placed on the sides of it, it will become flat or into a  shape or a  shape depending on the items surrounding it.

Uses
Glass panes are much more efficient than glass, as they can make windows of 267% more area than glass blocks, because only 6 glass blocks are required to make 16 glass panes. They appear somewhat brighter than glass blocks.

Because of their shape they can be used to displace water and lava to make an air pocket.

Also, both items and players can fit through 2x2 glass pane holes, so caution is advised when walking on them.

If used on the edge of a building as windows with no block above it the player can look quite far up to the sky.

A glass pane can be placed on top of a pressure plate. If surrounded by TNT, items can still be dropped onto the pressure plate as normal due to the hitbox. This also prevents the TNT from jumping to the side unpredictably which is useful for accurate cannons.

Glass panes, along with iron bars, can be used in wireless redstone setups due to a block update glitch.

History
Glass panes were introduced in Beta 1.8. Before Beta 1.9 Prerelease 2, glass panes were dropped as a resource when destroyed.

Before Beta 1.9 Pre-release 4, Glass Panes would not attach to many transparent blocks, mainly stone slabs, stairs, pistons, sticky pistons, farmland, iron bars, doors, signs, and fences.

Trivia

 * A 2x2 area made of glass panes can make a hole big enough for the player to fit through in the middle, but not rain, water, etc. this can be used as an easy entrance for an underwater house, and even an exit if pistons are used. Iron bars work in the same manner.
 * Be careful where you plant Saplings: when a tree grows, any Glass Panes that are in the way will be removed.
 * Glass Panes are much brighter than Glass Blocks, thus making them more suitable than Glass Blocks for high-visibility structures.
 * If two glass panes are placed with one empty space in between, a spider (which is 2 blocks wide) can still fit through.
 * You cannot make glass panes back into glass, so the transformation is irreversible.
 * You can stand on the same block that an unconnected pane is on, because the collision box is much smaller than normal blocks.
 * If placed by a door it will not connect, and it cannot connect to any objects that aren't full 8blocks (saplings, slabs, chests, etc.).
 * Glass panes, as well as iron bars, can be used for safe garbage bins, especially if the player is low on lava, because items can pass through holes while players cannot.