How many elbow T and cross pieces do you need to build a 20-by-20 grid?

To build a 20-by-20 grid, you would need 21 horizontal pieces and 21 vertical pieces, totaling 42 pieces. For the elbow T pieces, you will need one at each intersection, which amounts to 400 intersections (20 rows x 20 columns). Cross pieces are typically used at intersections as well, meaning you would also need 400 of those. Thus, the total requirements would be 21 horizontal pieces, 21 vertical pieces, and 400 elbow T and cross pieces each.