Category: Image - :Vinyl.png|thumb|100px|right|Chemical structure of the vinyl functional group.

A vinyl compound is any organic compound that contains a vinyl group (also called ethenyl), −CH=CH2. These are derivatives of ethene, CH2=CH2, with one hydrogen atom substituted with some other group.

A related phrase is vinylidene which stands for a double-bonded two-carbon residue with two rather than one substituents, for instance in 1,1-dichloroethene ("vinylidene chloride").

The vinyl group forms part of the allyl group and is also contained in all acrylates. Many alkenes contain a vinyl group.

The vinyl group can be made to polymerize, forming vinyl polymers. In these polymers, the double bonds of the vinyl monomers turn into single bonds and the different monomers are joined by single bonds. This is an instance of addition polymerization. There are no vinyl groups in the resulting polymer. It is also important to ascertain the absence of unreacted vinyl monomer in the final product when the monomer is toxic or reduces the performance of the plastic.

:This article is about the house covering. See rail siding for an alternative usage of this term.

Category: Image - :LightningVolt Corrugated Steel Siding.jpg|thumb|Corrugated steel siding, for the side of a barn.

Siding is the outer covering or cladding of a house meant to shed water and protect from the effects of weather. On a building that uses siding, it may act as a key element in the aesthetic beauty of the structure and directly impact its property value.

Siding may be formed of horizontal boards or vertical boards (known as weatherboarding in many countries), shingles, or sheet materials. In all four cases, avoiding wind and rain infiltration through the joints is a major challenge, met by overlapping, by covering or sealing the joint, or by creating an interlocking joint such as a tongue-and-groove or rabbet. Since building materials expand and contract with changing temperature and humidity, it is not practical to make rigid joints between the siding elements.

Siding may be made of wood, metal, plastic (vinyl), masonry , or composite materials. It may be attached directly to the building structure (studs in the case of wood construction), or to an intermediate layer of horizontal planks called sheathing.