As teachers express concerns about the way in which new GCSEs in England are being graded from 9-1 rather than A*-G, we answer some key questions about the changes.
When do the new 1-9 grades come in?
The new grades are being phased in, starting with some of this summer's exams.
New-style GCSEs in English language, English literature and maths will be taken by the current Year 11 students - these exams will be graded in the new way, with nine as the highest mark and one the lowest.
A four is broadly being compared to a C grade, although the exams watchdog, Ofqual, warns against "direct comparisons and overly simplistic descriptions".
For most other subjects - including biology, chemistry, physics, computer science, French, Spanish, religious education, geography, music and history - the new grades will be awarded from the summer of 2018.
A third wave of 9-1 graded GCSEs - including psychology, ancient history, business, information and communications technology (ICT) and media studies - will be taught from September 2017 with exams in 2019.
So some teenagers will have a mix of GSCEs under different marking schemes?
Yes, that's right. The current Year 11s will get English and maths results under the new numerical grading scheme and the rest of their options will be graded A*-G.
The current Year 10 students will then sit most of their GCSEs under the new system, but they might have some under the old system, for example if they are taking ancient history or ICT, while those pupils now in Year 9 will be fully "moved over" on to the numerical grading system.
How do the new grades compare to the old ones?
Grades nine, eight and seven are broadly equivalent to an A* and A. Grades six, five and four are in line with B and C grades. A three would be broadly similar to a D grade, with two and one taking in grades E, F and G.
There is still a U (ungraded) mark.
Education Secretary Justine Greening says a grade four will be seen as a "standard pass" and a grade five as a "strong pass".
She also says that for school performance tables, the government will publish "standard passes" (grade four and above) but also the "strong passes" (at grade five and above).
Watchdog Ofqual says that, broadly, the same proportion of teenagers will get a grade four and above as currently get a grade C or above.
It also says a formula will be used which will mean that about 20% of all grades at seven or above will be awarded a grade nine.
Chief regulator Sally Collier says students who get a nine will have "performed exceptionally".
Won't the first cohort to sit the new exams be disadvantaged?
It certainly feels like this and students in the next two year groups are guinea pigs for the new grading system.
However, Ofqual insists these students will not be disadvantaged.
It says that in 2016, in English and in maths, about 70% of 16-year-old students achieved a grade C or above and so it would expect a similar percentage to achieve a four and above in this summer's exams.
Ofqual also says exam boards will use test results from national curriculum tests (Sats) taken at the end of primary school to predict the likely achievement at the new grades of one, four and seven.
Why are GCSE grades in England being changed?
The new GCSE grading scheme is part and parcel of a new curriculum which was introduced in England's schools in 2014 by the then Education Secretary Michael Gove.
The new GCSEs courses include much less coursework than before, with grades in almost all subjects depending on exams.
Courses are designed to be more rigorous with exams taken after two years of study, rather than in modules with exams along the way.
What is happening in Wales?
Change is under way in Wales as well as in England. The Welsh government has introduced new and revised GCSEs taught from September 2015.
The most significant changes are in English language, Welsh language and mathematics. In all the changed subjects, the new or revised specifications will be the only ones available to state schools in Wales. They will be delivered by the WJEC examination board.
One crucial difference to England is that the established grading structure of A*- G is being maintained.
What is happening in Northern Ireland?
While pupils in England will have results graded 9-1 and pupils in Wales will have A*-G graded results, pupils in Northern Ireland could end up with a mix of A*-G and numerical grades.
Initially, the Northern Ireland government said all exam boards operating in the province must give their results on an A* to G basis. This led English exam boards OCR and AQA to announce they would not offer the new GCSEs there.
But in June 2016, this decision was reversed by new Education Minister Peter Weir and pupils will now be allowed to sit GCSE grades from English exam boards giving results using the 9-1 system.
Approximately three-quarters of GCSEs in Northern Ireland are taken through the NI Council for Curriculum, Examinations and Assessment (CCEA); the remaining quarter through the AQA, OCR, Edexcel or WJEC exam boards.
What about Scotland?
Scotland has its own system of public examinations: Nationals and Highers.