What is the area under a velocity-time graph and how can the total area be calculated?

The area under a velocity-time graph is the distance travelled in a specific direction – the displacement. And when the distance travelled has a constant, acceleration of deceleration, the graph can be divided into shapes e.g. triangles and rectangles/squares to calculate their total area. But when velocity-time graphs have accelerations and decelerations that are not constant, but rather curved lines, work out the total area under the graph by counting the number of squares. Then add the total of the parts (halves and quarters) of squares to include the total area.