Thanks to all. Yes, it is the fact: numer:false$ is(0.2=1/5); /* false - it's clear */ numer:true$ is(0.2=1/5); /* true - and it's clear too */ Now I'd like to compute the error: numer:false$ 0.2-1/5; returns 0.0 I think that the error must be about 1e-16. Can I compute it explicitly?