Decimals are written in word form by using a place value chart and using the place of the last digit on the chart to name the given number. Decimals on a place value chart are designated in tenths, hundredths, thousandths, ten-thousandths, hundred-thousandths, millionths and so on.
If a decimal is 0.500, then the written form is five hundred thousandths. This is because the number after the decimal is 500, and the final number, zero, lands in the thousandths place on the place value chart. This is not to be confused with 0.00005, which is written five hundred-thousandths. Both sound the same when spoken but are two different numbers.