There are around 500,000 serving teachers in England and Wales, yet the latest figures show overall teaching union membership is nearly twice that at 940,665.
How can this be? Because statistics collected by the Certification Office are not always quite what they seem. The latest figures confirm the National Union of Teachers as Britain's biggest teaching union with a 9 per cent increase in members compared with a 12 per cent drop for the NASUWT.
But by another, some say more important, measure - the number of fee-paying members - the NASUWT is gaining ground.
Even when you strip away non-fee paying members there is a difference of more than 200,000, between the number of teachers and union members, something the small Scottish and Northern Irish membership cannot account for. Every union leader says their figures are accurate, before muttering darkly about the dirty tricks used by rivals.
But fierce competition means a strong temptation to use every strategy to boost numbers.
It is a legal requirement to submit accurate figures but as one former general secretary said: "It's a bit like accounting. I'm not saying unions are like Enron but it is an inexact science."
Some explanations are obvious. Not all members are teachers. Some unions rely on support staff and FE and childcare sectors to top up the numbers.
Unions offer new members an introductory, free subscription. But because these members, in theory, will pay fees unions may include them in the fee-paying numebrs.
Then there are members whose fees are outstanding. Maybe they have forgotten to pay, do not want to be members or are dead?
The situation is even more difficult to read because of the different types of membership total.
The Certification Office publishes figures for overall and fee-paying members. Only the Association of School and College Leaders and Professional Association of Teachers submit identical figures for both. And for the big three classroom unions - the NUT, NASUWT and the ATL - there is a third alternative, the Trades Union Congress set of figures.
The former general secretary argues these are the most reliable because they are based on the relatively high per-member fee a union must pay for TUC affiliation.
"They are taken on trust," he said. "What union would pay hundreds of thousands just to show their membership had reached a certain level?"