Afetr the usual mixing, cutting the deck in half, turning one half then riffel shuffling, the cards end up mixed, some face-up some face down. Cards are picked off two at a time those that match (both face up or both face down are set aside. Those pairs comprising one face up and one face down are found to match in colour.

Can't figure out the initial set-uo, any help please?

If I understand you right, a mirror stack should do, here: from top down 13 red, 26 black and then 13 red cards e. g. ... Jan

Thx Jan, I will experiment with this.

If you have a fully alternating red-black deck, turn one half over and shuffle face up into face down with a standard riffle shuffle. Cut one card from top to bottom and you're set: all face-up/face-down pairs will be the same colour. pairs of face-up or pairs of face-down will not.

Hope that helps.

L

Thank you Loz.