Back before computers were invented Librarians put information about their books on cards and filed those cards in cabinets called Card Catalogs. Multiple copies of the card for a particular book were printed, and particular bits of information–such as the title, the author’s name, or a subject the book was about–would be added at the top of different cards. The cards were then filed in alphabetical order in the card catalog so people who wanted to find a book (patrons) could search and find the book they wanted. The people who decided what to put on the cards were called Catalogers. The information on the cards is human created metadata.
The Librarians had a special ‘Librarians only’ card catalog called a Shelf List. This cabinet had all the cards filed in the same order as the books on the shelf. If they needed to mark a card in that catalog they used little metal clips on the top of the card. There would be a list of what each color clip meant posted on the side of the cabinet or on the first card in a drawer. The list might say: Black = missing; Blue = being repaired; Green = on reserve for class; Yellow = Additional copy on order. When they began moving the information from cards to computers the librarians used color clips to show which cards had already been copied and which ones still needed work.
Sometimes the Librarians needed to let their patrons know something special about a particular book, like a special location, but they didn’t want to do anything to the card that would require them to change or replace it when the location changed. For those times they put plastic covers with a colored stripe at the top over the card to show there was something special about it. There would then be a sign that told the searcher what the colors meant: Red = Reference shelves; Green = Children’s collection; Blue = 3rd Floor reading room.