 In practice , measurements are carried out on a distance known as <unk> ( Si ) , which is the distance between the planes denoted by the Miller indices { 220 } , and is equal to a / √ 8 . The 2006 CODATA value for <unk> ( Si ) is 192 @.@ <unk> ( 50 ) pm , a relative uncertainty of 2 @.@ 8 × 10 − 8 , corresponding to a unit cell volume of 1 @.@ <unk> ( 13 ) × 10 − 28 m3 . 
