The array can be shown as 5 groups of 3 showing that 5 x 3 = 15. Or it could be shown as 3 groups of 5 showing that 3 x 5 = 15. This demonstrates that multiplication is commutative. Image caption, ...

Abstract: Transverse waves offer shorter wavelengths and higher spatial resolution than longitudinal waves at identical frequencies, providing advantages for solid nondestructive testing (NDT).