What is "black rubric"

The term Black Rubric is the popular name for the declaration found at the end of the "Order for the Administration of the Lord's Supper" in the Prayer Book of the Church of England ( BCP) which explains why the communicants should kneel and excludes possible misunderstandings of this action. The term dates from the 19th century when the medieval custom of printing the rubrics in red was followed in editions of the BCP and the declaration was printed in black; but the Declaration itself was composed in 1552.