X and Y mark the spot. The seventh title in the popular Sir Cumference series introduces the mathematical concept of coordinate geometry in a unique and memorable way. While riding through the forests of Angleland, cousins Per and Radius realize they are lost and are desperate for a map to guide them home. Soon they come across a mysterious house in the hills. Inside they find a map to a treasure belonging to Xaxon Yellowbearyd, the fiercest Viking warrior of his time. Per and Radius must decode the strange numbered grid on the map, while trying to steer clear of the pack of bungling bandits who are on their tail. Will they find the treasure in time? Cindy Neuschwander delivers yet another intriguing math adventure featuring the well-known characters of Angleland. Readers will enjoy following Per and Radius in their quest while learning how coordinate geometry relates to everyday life. Wayne Geehan's beautiful illustrations offer a bit of comic relief, while also clearly demonstrating the concepts explored in the story.