I'm building a foundation wall for an addition to my house. Code requires 12 inch block underground. I had planned on switching to 6 or 8 inch block once I got above grade. What happens to the mortar joint at the transition from 12 inch block? With regular block, you would only have mortar along the outside edge, and possibly on the webs if it lines up right. Is there a special way to mortar this transition? Thanks...

Aren't all your blocks filled with mortar and horizontal/vertical rebars added per local building code? Your above ground block courses would sit on the mortar filled blocks.