Most real-world quantities don’t depend on just one thing. Temperature changes with both latitude and time of day. Profit depends on sales price and production volume. To handle these situations, calculus grows a new dimension—literally. In this chapter, we extend the ideas of derivative and optimisation to functions that depend on two, three, or more variables. Don’t worry: the core intuitions you already have will carry over, just with a few extra coordinates along for the ride.