Aha, found how they were used. They were placed outside the person's house for the twelve months of mourning, and only then transferred to the church. As the custom died out in the nineteenth century, now we only ever see them in churches.

The arms were painted on a diamond-shaped board with a black background for a single or widowed person. For someone leaving a relict, it was half black and half white, black behind the dead person's half of the arms and white behind the survivor's.