if not 0.0, were all seen to be (1.11022302463e-16)*2 n . Is there anything in particular about 1.11022302463e-16? It turns out that it's the machine's epsilon (http://en.wikipedia.org/wiki/Machine_epsilon) (also see machine's epsilon ). What is 1.11022302463e-16 in binary? dennis:~# echo "obase=2;1.11022302463*10^-16" | bc -l .0000000000000000000000000000000000000000000000000000011111111111111 If you round this up by one