Once a common site in churches everywhere, these crosses are often today found only as outlines in the stonework, and as such are sometimes mistaken for graffiti.
When a church was first constructed (or significantly altered), it was consecrated by the local Bishop prior to worship taking place. The Bishop would bless the building and anoint it with holy oil, twelve times outside and twelve times inside, each location being marked with a cross.
When originally created each of these crosses would have been painted, most usually in a red pigment, and surviving examples can be found in many churches across England. However, in most churches the pigment has now been lost, leaving only the scored setting-out lines of the crosses visible.