The Consecration Cross, also called a Rounded Cross, takes several forms and here we see two common examples. The arms of the upper version are splayed like paws (known in heraldry as pattee) making it similar to the St. John's Cross. The lower one is the same design as the Sun Cross used in Pagan symbology.1
Twelve of these crosses were carved or painted on the walls, pillars or altars when churches were built in medieval times; each cross representing one of the twelve apostles. The crosses were positioned at the various places that had been anointed with blessed water or chrism2 during the consecration of the building. Typically, a sconce is positioned just beneath each cross to hold a candle which may be lit on certain festal occasions. It is believed the tradition was started by Emperor Theodosius in the 4th century, who purified Pagan temples for Christian worship.
Bishops conducting the inauguration ceremony might have initially used chrism to draw the cross at the various points in the church, and then later more permanent crosses would be painted or carved. Examples of these crosses have survived longer than wooden or metal crosses, which have dropped off or been stolen over the years.
The crosses were placed inside the church, three on each of the four walls. Alternatively, twelve critical points were selected; for example on main supporting pillars, on a wall facing the altar or on the altar itself. This was to show that whilst the church is built for the worship of God, the twelve apostles are invoked to support the building.
Some Celtic churches additionally have Consecration Crosses on the outside of buildings. Leabhar Breac ('The Speckled Book'), written in a mix of Latin and Irish, describes how the bishop marks crosses on the external buttresses with a knife3. External Consecration Crosses can be seen on the 750-year-old Salisbury Cathedral in England.
The crosses have equal length arms to distinguish them from the main crosses of the church, which would typically have longer lower portions. The main crosses symbolise Christ's Crucifixion, whereas the Consecration Crosses symbolise each of Christ's disciples. (See also the Apostles' Cross.)
If you see a relatively new-looking Consecration Cross on the wall of a cathedral, it may well be a Canterbury Cross.