If you’ve got columns and you’ve got the cross bar on top called the entablature then you need the cherry on the cake – the triangular bit on the top called the pediment.
The flat bit is called the tympanum and it was a very useful space to decorate with sculpture which explains a bit about the building. Like in this 19th century building in Bermondsey – The Hop Exchange – all about the growing, harvesting and the use of hops.
The one on the British Museum tells of the civilising effect of knowledge.
The source of many of these buildings is, of course, The Parthenon. Ironically, inside the British Museum we have a display of sculpture that was ripped off The Parthenon by not so civilised Brits, such as these from the east pediment, which tell of the birth of the goddess Athena. You can tell they are meant to fit in a triangle (these are half of the whole).
Modernism eschewed the classical orders for something quite different but Post Modern architecture gives the odd nod to the past as in this high rise by Philip Johnson c 1984.
So keep your eyes open – it may not be obvious, but the Classical Orders are all around us in our lovely city.