The line of intersection of two planes is perpendicular to both planes' normal vectors [math]\mathbf{n}_1[/math] and [math]\mathbf{n}_2[/math] and therefore parallel to [math]\mathbf{n}_1 \times \mathbf{n}_2[/math]. Turning this around, [math]\mathbf{n}_1\times\mathbf{n}_2[/math] is a vector parallel to the planes' line of intersection. [br][br]Use this interactive figure to visualize and explore the intersection of two planes. Click on the "Hide planes" check box to hide the planes and see only the line of intersection, the two unit normal vectors, the position vector of point [math]G[/math] in the line of intersection, and the unit vector [math]\mathbf{n}_{\text{ABC}}\times\mathbf{n}_{\text{DEF}}[/math].

[i]Developed for use with Thomas' Calculus, published by Pearson.[/i]