Pupils may think that ‘the biggest number has to go on top’ when they set out addition calculations. Whilst this is not incorrect, it can highlight errors in comparing decimals when working with such numbers. For example, a pupil may set out:

as

or

Both ways of setting out this calculation show that the pupil is judging 1.86 to be larger than 7.5 and although the correct answer will be obtained in the first example, a similar misunderstanding can cause more difficulty and confusion in subtraction.