Pen + Gear Graph-Ruled Index Cards. They’re a bit on the thin side, but they take ink well, without feathering or bleeding through. And they’re printed with a very fine light-blue grid (five squares to the inch) that doesn’t get in the way of what one is writing or drawing or mapping. These cards are much better than Oxford or Staples grid cards, and a fraction of the cost of Exacompta: 48¢ for 100 cards. Highly recommended.