The comparability of examinations in different subjects has been a controversial topic for many years and a number of criticisms have been made of statistical approaches to estimating the "difficulties" of achieving particular grades in different subjects. This paper argues that if comparability is understood in terms of a linking construct then many of these problems are resolved. The Rasch model was applied to an analysis of data from over 600,000 candidates who took the General Certificate of Secondary Education (GCSE) examinations in England in 2004. Thirty-four GCSE subjects were included in the final model, which estimated the relative difficulty of each grade in each subject. Other subjects failed to fit, as did the fail grade, U. Significant overall differences were found, with some subjects more than a grade harder than others, though the difficulty of a subject varied appreciably for different grades. The gaps between the highest grades were on average twice as big as those between the bottom grades. Differential item functioning (DIF) was found for male and female candidates in some subjects, though it was small in relation to variation across subjects. Implications of these findings for various uses of examination grades are discussed. (Contains 3 tables, 2 figures and 7 notes.)