I use these cards for everything--we just created a "cook book" that is actually a 40 card set of recipes that can be used on their own and can go into a recipe file so easily.
I use these index cards to send information to our customers. I usually have a hard time finding ones that are unlined but Staples always carries a variety of cards for every need. The Staples brand are good quality and acceptable to the USPS. I would like to have the unlined cards available in colors but for now the white works fine for our needs. I appreciate the help that is given by the associates since sometimes these cards are hard to find. It is very helpful to be "taken" to the location of the item you are looking for rather than just being directed to "Aisle 9".
I use the 4" x 6" white index cards for some monthly printing and advantage of a postcard cost over letter cost in postage. Now if only I might find them UNlined and in color as well...
Stock too thin for my intended purpose but no way to determine that without buying 500 cards (even though I only needed many fewer). Packs of 100 would have been better.