Some people call them mobile learning units. Brian Cooklin, president of the Headteachers' Association of Scotland, calls them huts.
However, even huts are a luxury some schools find themselves unable to indulge in, according to evidence submitted to the Parliament's education committee last week.
Figures from the Association of Directors of Education in Scotland not only showed how much reducing class sizes to 18 in P1-3 would cost 22 councils (more than pound;400 million apparently). They also highlighted the number of primaries for whom expansion is simply not an option: 407, or roughly one third of the 1,420 primaries that were surveyed.
"Things like lack of space in the playground meant they simply could not expand," Murdo MacIver, convener of the association's resources committee, told The TESS.
No huts for them then.
So if councils fail to make year-on-year progress towards class sizes of 18 in P1-3, what "action" should the Government take, SNP MSP Rob Gibson wanted to know.
They should not be "clobbered", Mr Cooklin replied.
Predictably, Mr MacIver, who is also head of education provision in North Lanarkshire, agreed (although he put it more delicately, saying he didn't think the Government should "impose or insist", but "take account of the individual circumstances").
An appropriate action, Mr Cooklin went on to suggest, might be more money. Anyone who believed that reduced classes would "blithely happen" was "a bit naive", he concluded.
Apart from money, there was also the pesky matter of the law, it transpired. Classes of 18 might be an aspiration but, legally, the maximum is 30. In North Lanarkshire, Mr MacIver told the committee, they were attempting to drop classes from 25 to 23 in P1 from August, but already a placing request from a parent had been upheld by the council. "We are facing the unknown," he warned.