13”Crochet Blocking Board - Upgraded Style with 4 Tension Rods and 20 Pegs – Blocking Boards for Crochet Projects, Ideal for Granny Square Card Deck Users （Natural Colour）

I've been making c2c squares to use for a blanket out of cotton, and I bought a wooden blocking board because I didn't have anything suitable to make one. It is wooden and has holes every half inch and pegs to hold the piece in place. I use my steamer and steam both sides and let it dry. Here's the one I bought. I've seen a lot of pictures of people using those interlocking foam tiles. The one thing I wonder is what do they use to mark the measurements to be absolutely sure it doesn't get on the yarn.