BHA normally comprises of all the string ran in hole below the drill pipe while drilling and they could be made up of a drill bit, bit sub, near bit or string stabilizer(for directional control, gauging the hole size and help to stabilize string), float sub(contains flapper that ensures one way flow and useful for well control), drill collar(providing weight on bit), mud motor(using rotor-stator arrangement for directional control), rotary steerable motor(using activation pads for directional control), MWD tools (for inclination and depth(MD, TVD) measurement), LWD tool(contains radio active material and used for measuring the resistivity, density, neutron porosity, sonic etc) mainly for lithology and fluid content measurement, Gyro(used for depth and inclination measurement when magnetic interference is expected as conventional MWD give erroneous measurement when there is magnetic interference), hole opener assembly.e.t.c.