A joint or articulation (or articular surface) is the location at which bones connect. They are constructed to allow movement (except for skull, sacral, sternal, and pelvic bones) and provide mechanical support, and are classified structurally and functionally.

Classification

Joints are mainly classified structurally and functionally. Structural classification is determined by how the bones connect to each other, while functional classification is determined by the degree of movement between the articulating bones. In practice, there is significant overlap between the two types of classifications.

Terms ending in the suffix -sis are singular and refer to just one joint, while -ses is the suffix for pluralization.

An articulate facet is generally seen as a small joint, especially used when speaking of the joints of the ribs.

Structural classification (binding tissue)

Structural classification names and divides joints according to the type of binding tissue that connects the bones to each other. There are three structural classifications of joints:

A building joint is a junction where building elements meet without applying a static load from one element to another. When one or more of these vertical or horizontal elements that meet are required by the local building code to have a fire-resistance rating, the resulting opening that makes up the joint must be firestopped in order to restore the required compartmentalisation.

Qualification requirements

Such joints are often subject to movement, as a function of the building's design basis. For a sample certification listing indicating fire tested motion, click here. Firestops must be able to demonstrate the ability to withstand operational movement prior to fire testing. Firestops for such building joints can be qualified to UL 2079 -- Tests for Fire Resistance of Building Joint Systems.

Whether or not the building elements forming the joint have a fire-resistance rating, the joint design must still consider the anticipated operational movement of each joint. Timing is also important, as freshly poured concrete shrinks particularly during the first few months of a new building, potentially causing joint size changes.

In audio engineering, joint refers to a joining of several channels of similar information in order to obtain higher quality, a smaller file size, or both.

Joint stereo

The term joint stereo has become prominent as the Internet has allowed for the transfer of relatively low bit rate, acceptable-quality audio with modest Internet access speeds. Joint stereo refers to any number of encoding techniques used for this purpose. Two forms are described here, both of which are implemented in various ways with different codecs, such as MP3, AAC and Ogg Vorbis.

Intensity stereo coding

This form of joint stereo uses a technique known as joint frequency encoding, which functions on the principle of sound localization. Human hearing is predominantly less acute at perceiving the direction of certain audio frequencies. By exploiting this characteristic, intensity stereo coding can reduce the data rate of an audio stream with little or no perceived change in apparent quality.

More specifically, the dominance of inter-aural time differences (ITD) for sound localization by humans is only present for lower frequencies. That leaves inter-aural amplitude differences (IAD) as the dominant location indicator for higher frequencies. The idea of intensity stereo coding is to merge the lower spectrum into just one channel (thus reducing overall differences between channels) and to transmit a little side information about how to pan certain frequency regions to recover the IAD cues.