Linked portal door

It's a square portal that leads seamlessly to another. The player can shoot the portal gun through it, but portals viewed through it don't look quite right.

The entity is used in every Puzzle Maker level, to connect the exit and entrance elevators to the entrance and exit airlock rooms; but also in the GlaDOS wakeup sequence and in Chapter 9, in the Chamber 75 Death Trap with 6 defective turrets aiming directly at you. The incinerator shaft and the turret room are actually completely separate from their supposed surroundings. They are the only uses of this entity in the final version of the game, though commentary states that it was often used during the development of the title to link chambers together.