After some research, I landed on the Stockroom+ Grid Ruled Index Cards (4×6 Inches, White, 300 Pack), which I purchased for $12.99 from Amazon.
This is a great quality index card for one of the best prices/values for this size and grid ruling.
It’s a noticeably thicker index card stock compared to most (~0.0106″ compared to Oxford’s 0.0072″ or Amazon Basic’s 0.0078″). Most of my pens and pencils weren’t visible through the Stockroom+ card even when held up to a standard room light, something which I couldn’t say about the thinner Oxford or Amazon cards. Only a thick Sharpie pen was slightly visible held up to a light, but none of them came remotely close to bleeding through. These cards write smoothly and take fountain pen ink well without any feathering.
The grid is 1/4″ and printed on both sides. The lines are a light grey which doesn’t overwhelm black ink lines in the 0.4-0.5mm range. The printed grid is also very standard from card to card, so if you need to line the patterns up or are OCD, you’re not going to have problems. The second line down matches up well with the top red lines on most other standard index cards.
I got mine for about $0.043/per card, so it’s a fantastic value.
I can’t wait to use them more and expect them to hold up incredibly well over time. This should be useful given that some of the math, engineering, and science topics I’ll use them for are some of the more well-traveled pathways in my slip box.
Have you used 4 x 6″ grid index cards that you like? I’d be curious to hear comparisons.