$MachineEpsilon
x = 0.0000000000000001
% + 1
% - 1

As expected the final result equals zero, since x < MachineEpsilon.

However, if I double x, then the final result should in my understanding still be zero, since x is still smaller than the MachineEpsilon, but it isn't zero. Question: why is this?

Note 1: If I triple x, then x is larger than the MachineEpsilon and indeed the final result is bigger than zero.

Note 2: On my computer the value for MachineEpsilon is $2.22045\times10^{-16}$ Is this the same as on any computer using Mathematica by the way?