Thanks to all who've replied; Carl's explanation in particular was very interesting. So the precision, suggested by the many decimals in the 'show', is not the actual precision the user should 'count on'. If you take 1/60 of a degree to be approximately 0.0003 radians, you should not use sin for smaller values. In all cases the actual precision of sin appears to be 4 to 5 decimals, and results should be rounded to that before using them. Now I'm wondering about cos, tan and also the inverses, asin etc. :-) Regards, Hans van Thiel Hugs> sin (1.000 * pi) 1.22460635382238e-16 Hugs> sin (0.999 * pi) 0.00314158748587949 Hugs> sin (1.00001 * pi) -3.14159265309255e-05 Hugs> sin (0.99999 * pi) 3.14159265307264e-05 Hugs> sin (1.001 * pi) -0.00314158748587925 Hugs> sin (0.999 * pi) 0.00314158748587949 Hugs> sin (1.0001 * pi) -0.000314159260191213 Hugs> sin (0.9999 * pi) 0.000314159260191458 Hugs> sin (1.00001 * pi) -3.14159265309255e-05 Hugs> sin (1.0001 * pi) -0.000314159260191213 Hugs> sin (0.9999 * pi) 0.000314159260191458 Hugs> sin (1.00001 * pi) -3.14159265309255e-05 Hugs> sin (0.99999 * pi) 3.14159265307264e-05 Hugs> sin (6.0001 * pi) 0.000314159260189269 Hugs> sin (5.9999 * pi) -0.000314159260190738 Hugs> sin (6.00001 * pi) 3.14159265298692e-05 Hugs> sin (5.99999 * pi) -3.14159265313387e-05 Hugs> sin pi 1.22460635382238e-16