The advent of the chimney spelled the introduction of what are now referred to as masonry fireplaces — that is, fireplaces, typically made of brick (although occasionally of stone), built directly into the wall of a building. In its essentials, the modern domestic masonry fireplace is not especially different from its medieval forebears. At bottom is the hearth, on which the fire burns. Surrounding the hearth is the firebox, which is the only section of the fireplace actually visible to a person sitting indoors. The firebox leads, along a slope, to the smoke chamber, which is designed to compress exhaust gases gently, without eliciting a backdraft. From the smoke chamber, the exhaust gas travels upward, through the flue, or the chimney proper, and into the atmosphere. Occasionally, on the very top of the chimney one finds a chimney pot, an elongated cap that both looks distinguished and serves the practical purpose of deterring inquisitive pests.