Altar crucifix

An Altar Crucifix or Altar Cross is a cross placed upon an altar, and is the principal ornament of the altar.

The first appearances of a cross upon the altar occurred approximately in the 6th century, although it remained unusual for several centuries, and even discouraged. When it was used, it seems to have been only during actual services, and probably a processional cross which was detachable from its staff, and placed on the altar after processing. This would at first almost always a cross rather than a true crucifix; these began to be made specifically for altars in the late 11th century, and became more common from the 12th century, though they may have been expensive at first. By the start of the 13th century, treatises by Pope Innocent III expect there to be a cross between two candles on the altar during the mass. This period was also the era when candlesticks, also probably carried in procession at the start of a service, started appearing upon altars instead of nearby, and as such marked a rather large evolution in the adornment of altars. Around the 14th century, altar crosses were almost universally replaced by crucifices, probably now affordable by all churches, however, it was not until the Roman Missal of Pius V in 1570 that there is any mention of an obligation to have the crucifix on the altar.

Early Christians were not accustomed to publicly expose the cross or crucifix due to fear of subjecting it to the insults of pagans or scandalizing the weak. To avoid this, they often used symbols like the anchor or trident.

The crucifix placed upon the altar is intended to serve as a reminder to the people in attendance and the celebrant of the believed nature of the Eucharist as the actual body of Christ; the altar itself symbolically represents Golgotha. It is for this reason that Roman Law decrees it necessary to have the crucifix upon the altar whenever Mass is celebrated. Specifically, it is placed directly in between the Candlesticks in such a way that it is conveniently seen by the people. In some cases, to better fulfill this requirement, the crucifix is instead hung on the wall behind the altar, so that when the priest is facing the congregation the crucifix is not obstructed. In some churches the crucifix is suspended mid-air via strong, nearly invisible metal cords, directly above the altar itself.

...
Wikipedia