Errors in the text could also be corrected in the margins. Corrections made in this manner are indicated with matching symbols. Before the 10th century the matching symbols would be HD and HS, latin abbreviations meaning look down in the margins or look up in the margins. The matching correction would then marked with either HD or HS depending again on if the correction was above or below the place where the reader should look for its mate. Later these would change into matching signs “They were especially suitable when several corrections occurred on the same page because the reader could swiftly locate in the margin the match” (Graham and Clemens 37).

In the manuscript on the right you can see a key like symbol at the end of a line of text.  It matches the symbol next to the correction written in the bottom margin.