The powder method is used to determine the value of the lattice parameters accurately.
Lattice parameters are the magnitudes of the unit vectors a,
b and c which define the unit cell for the crystal.
If a monochromatic xray beam is
directed at a single crystal, then only one or two diffracted beams may result. 

If the sample consists of some tens
of randomly orientated single crystals, the diffracted beams are seen to lie on the
surface of several cones. The cones may emerge in all directions, forwards and backwards. 

A sample of some hundreds of
crystals (i.e. a powdered sample) show that the diffracted beams form continuous cones. A
circle of film is used to record the diffraction pattern as shown. Each cone intersects
the film giving diffraction lines. The lines are seen as arcs on the film. 

For every set of crystal planes, by chance, one or more crystals will be in the correct
orientation to give the correct Bragg angle to satisfy Bragg's equation. Every crystal
plane is thus capable of diffraction. Each diffraction line is made up of a large number
of small spots, each from a separate crystal. Each spot is so small as to give the
appearance of a continuous line. If the crystal is not ground finely enough, the
diffraction lines appear speckled.
This arrangement is achieved practically in the Debye
Scherrer camera illustrated here...