Monday

Rocket is on his knees with masking tape, laying lines across the garage floor. "A grid. One square is one floor tile."

"Every corner of the clubhouse gets a name," Raven says. "Two numbers. Across, then up."

She reads from the Blueprint Book. "Main room: (1, 1), (7, 1), (7, 5), (1, 5)."

Rocket runs string from corner to corner. A rectangle appears on the floor. "How long is the front wall?"

"What do you notice about (1, 1) and (7, 1)?" Raven asks.

"Same up number," Rocket says. "So the wall runs straight across. From 1 to 7. Six squares."

Nova hovers above the string, her light sliding along the front wall. "You did not need the tape measure," she says. "Only the coordinates."

Every point on the grid has coordinates (x, y). The x-coordinate says how far across from the origin, the y-coordinate how far up.

The main room has vertices A (1, 1), B (7, 1), C (7, 5) and D (1, 5). Join them in order and you get a rectangle.

A vertex is a corner. To draw a polygon from its vertices, plot each point, then join them in the order given.

A (1, 1) and B (7, 1) have the same y-coordinate, 1. The side AB is horizontal. Its length is the difference of the x-coordinates: 7 - 1 = 6.

B (7, 1) and C (7, 5) have the same x-coordinate, 7. The side BC is vertical. Its length is 5 - 1 = 4.

Same x means a vertical side. Same y means a horizontal side. Subtract the coordinates that differ, and you have the length.

Corners named, walls measured. Tomorrow you find perimeter and area from the coordinates, and cross into all four quadrants.