curve widht ?

i tried to find some doc on wiki for curve wiht but not luck

like on widht
whne i trioed to add some withd na poly curve
i can see it move up and down but no width

so what where does this widht apply is it only for surface curve may be ?

now i can see that extrude does give a widht at 90 degress to the curve
which i can alway work with !