index cards are small paper or cardboard sheets, roughly A6 in size that were (are?) used to store atomic information.

They were used as catalogues in libraries and for personal knowledge management before computers.

In Notes on designing books by Derek Birdsall, the author discusses how he used A6 index cards to plan and organise a book before computers came around.

Index cards can also be recreated or represented in the computer and it’s often useful to use their concept also in the digital world.

See hypermedia, hypertext, zettlekasten, libraries, Derek Birdsall