For more than two decades GCSEs have provided teachers, pupils, parents and employers with a universal, easily understood way of measuring the academic achievement of 16-year-olds.
Like any currency, the qualification has not been immune from the ravages of inflation. If the increase in top grades was not enough to prompt a fall in the GCSE's market value, the rise in the number of pupils staying on to take more advanced qualifications surely has. Whatever the rights and wrongs of the annual debate over ever-improving results, there is little doubt that a GCSE is worth less today than it was in 1988.
Nevertheless, it continues to function as an essential means of exchange. Pupils can utilise it - indeed, they need it - to help them win jobs and places in sixth forms, colleges and universities. And despite their grumbling, employers, educational institutions and politicians all continue to accept it as a measure of attainment.
Why? Because the GCSE has critical mass. Everyone understands and knows what a GCSE means because almost everyone uses it. It is a common currency.
Now all that could be about to change. Three interlinked events in the last month suggest schools in England could be on the verge of an exams revolution.
First came the Government's long-expected decision to axe the Qualifications and Curriculum Development Agency, which was about much more than getting rid of a quango to save money.
The agency's curriculum functions will be taken back in-house to the Department for Education. But when it comes to its role in developing the criteria for exams like the GCSE, Education Secretary Michael Gove wants a completely new approach.
He said: "My view is that Government or its agencies should in principle not be involved in this kind of work in the future, which is properly the domain of awarding bodies."
In other words, a total state withdrawal. Ofqual, the exams watchdog, will continue to regulate the system, but it will be exam boards, assisted by universities at A-level, that now decide its future direction.
Just days later came the news that all schools would be allowed to offer IGCSEs as alternatives to the conventional GCSE in all subjects.
Ministers said giving state schools the freedom to use the more traditional O-level style exams would help them close the "already vast divide" with the independent sector. But the move is even more significant because it breaks the compulsory link between the exams available to state schools and the national curriculum.
GCSEs have always been tied to the curriculum programmes of study set by Government and its agencies. This was why the previous Labour government withheld permission from state schools to use all IGCSEs. It was nothing to do with the quality of the exams - ministers had already given nine of them the go-ahead. But they were concerned that some IGCSEs in core national curriculum subjects such as maths, English and science did not adequately cover the programmes of study.
The Government has no such qualms and has given these core IGCSEs the green light without requiring any changes. With the strict national curriculum requirement out of the way, state schools will be free to choose from any qualifications in any subject that exam boards decide to offer.
Currently, the qualifications have to be approved by Ofqual. But even that could change if the Government decides to follow the recommendations of the Sykes review of the future of the English qualifications and assessment system in schools, which was commissioned by the Conservatives in opposition. "It should not be the regulator's job to determine precisely which qualifications a school, college or other provider may offer," it concluded.
Early signs of how this deregulation might play out emerged last month when The TES revealed that the Assessment and Qualifications Alliance (AQA) was developing gender-specific alternatives to the GCSE, tailor-made for boys and girls.
The board is the only one of the big three awarding bodies not currently offering IGCSEs. Now, obviously aware of a market ready to take off, AQA is coming up with its own variation on the GCSE format.
Teaching unions are alarmed at the prospect of this free-for-all. John Dunford, general secretary of the Association of School and College Leaders, said: "Exams are not products on a store shelf. They determine young people's futures and should not be subject to market pressures."
He argues that allowing state schools to introduce alternatives to GCSEs will "increase uncertainty for parents, pupils, employers and the general public".
Outside the core national curriculum subjects, that uncertainty has already arrived. Secondaries have long been ranked according to the benchmark of the percentage of their pupils gaining five A*-C GCSEs or equivalent, including English and maths.
But behind that straightforward-sounding definition lie all sorts of variations. The keyword is "equivalent". Many vocational and other non-GCSE qualifications are, for league-table purposes, equivalent to one, two, three or four GCSEs, despite many requiring much less teaching time. Figures released by the Government last month reveal that two-thirds of all state-funded secondaries have at least part of their benchmark score made up of one of these equivalents.
Vocational qualifications have been offered by some state secondaries for decades. Schools argue that they are necessary to meet and recognise less academic pupils' needs and aptitudes, allowing everyone to stay engaged and leave with qualifications.
But questions remain over schools' motives for offering some of the most popular of these qualifications, which critics deem "highly questionable" and "pseudo vocational".
The OCR National Level 2 in ICT is one of two ICT qualifications deemed by Ofsted to be of "doubtful value" because, it says, they teach pupils what they already know and neglect essential skills. But it is used by more than half of secondaries, making it the fourth most popular 14-19 course in English schools.
It is easy to see what attracts schools to a course that can be worth up to four good GCSEs in the league tables and requires much less teaching than its GCSE league table equivalent. Heads defend their choice, arguing that the qualification offers pupils essential life skills. But critics ask whether digital natives need to be taught how to use Microsoft Word, search the internet and reply to emails and what worth and currency such qualifications will have for colleges, universities and prospective employers.
They also lack the generic, easily recognisable title of a GCSE. There is plenty of anecdotal evidence to suggest that schools are not always open in telling pupils that they are being entered for one of the plethora of vocational alternatives to GCSEs.
I visited a comprehensive in the north of England where many pupils were encouraged to take some of these alternatives. The fact that the qualifications were not actually GCSEs did not prevent everyone - teachers and pupils alike - from referring to them as such.
"Pupil X was on the point of being excluded but will now leave with seven GCSEs," I was told. But this wasn't the case. It was as if the worth of the qualifications to the school in the league tables had magically transformed them into GCSEs, thereby also enhancing their value to the pupil.
Some pupils told me they had been encouraged to take a vocational science qualification and only discovered later that they could not use it to get onto A-level science courses at the local sixth-form college.
The Sykes review team came across similar incidents, prompting the Conservatives to suggest removing vocational qualifications from academic league tables altogether.
Ministers say they want pupils "entered for the qualifications that are in their best interests" rather than "simply to boost the league table position of the school". Mr Gove has also said he wants to preserve the GCSE. However, both of these goals could be compromised by the market forces his Government is unleashing.
There is no suggestion that colleges, universities or sixth forms will not recognise IGCSEs or the alternatives that AQA is developing. But employers may struggle to keep up with a fragmenting exam market.
That will not be a problem for schools - and they will doubtless argue that they choose the options that best suit their pupils. But new qualifications may be of little use to some pupils if prospective employers have never heard of them.
The more alternatives there are, the less likely they are to work like the GCSE - as a common currency understood by society at large.