Index cards have become flimsy and diffcult to use in my opinion. What has happened to the index cards of old? These are so thin its as if someone has cut regular paper to index cards size. should be called index paper not cards.
Paper quality was a bit thin but worked well for my purpose as I was not trying to write on both sides. If using a dark marker it would show through on the back side.
Other index cards are too thin and they are not good for studying with. They are durable and can take a beating when they are carried around in a bag all day. They do not keep away bears but you probably already guessed that...