> correct rational is 142857/1000000 That would be correct if the underlying representation were decimal. If it's binary, as it is in most Common Lisps, (ra

> correct rational is 142857/1000000 That would be correct if the underlying representation were decimal. If it's binary, as it is in most Common Lisps, (rational 0.142857) --> 9586971/67108864 because the underlying representation is binary, and that denominator is 0x4000000. My original comment about the representation being large enough to capture more bits than you specified was wrong; the unexpected behavior comes from the internal binary representation and the fact that 9586971/67108864 cannot be reduced. If you start with integers you get (/ 142857 1000000) --> 142857/1000000 so you could write a version of #'rational that gives the expected base-10 result, but you'd first have to convert the float to an integer by multiplying by a power of 10.