I have read on the Copic website (www.copicmarker.com
) that it does not matter how you store them -- vertically or horizontally. Also, I agree with many that oozak.com is The Best Place to purchase Copics. I recently bought the 144 Ciao markers in 2 sets of 72. They were sold out of one set but called me to let me know they had the individual colors to make that set up -- including the sleeve and the plastic case! I agreed (like I could refuse?!?) and they shipped both sets that day -- I had in 3 days! Free UPS shipping. Color me happy!