Card wire carries fiber through the card. The top picture shows a strand of card wire. Notice that it consists of a base wire with teeth on top. Every roll in a card that transports fiber is covered with card wire. Each roll is wound with one continuous strand of wire. Notice that the teeth have direction. The lower photo shows the teeth side by side on the surface of a roll. The density of the teeth is measured by points per inch and threads per inch. In the top photo we could count the number of points per inch along the length of the wire. In the bottom photo we could count the number of threads per inch across the surface of the roll. Multiplying points per inch by threads per inch gives you the number of points per square inch.