There are several of these sprinkled around London, but the one at Russell Square is particularly well-preserved and prominent. Embedded in Leslie Green’s stubbornly perfunctory and visually peakish facade, the logo radiates freshness and imagination like sunlight sneaking through a crack in the Berlin Wall. An interloper from the drawing board of a visionary rather than a functionary, it can’t help but catch and retain the eye.

What is it that makes, that made, this logo so potent, so intriguing?

Its antiquity, for starters. It has a quality of being ancient, of hailing from the far distant past. Yet it’s hard to tell from how far back it belongs. This gives it a flavour of mystery as well as heritage. Could it be before or after the war? And which war? Is it even the 20th century?

There’s also something a bit exotic about it. It’s a typeface that doesn’t feel instinctively native to the UK. Where has it come from? Where has it gone to? It’s not passed into the common lexicon. You won’t find it adorning anything new – or anything relatively new either.

But above all, it’s enormously alluring. This is a typeface to fall in love with. It knows it’s attractive, but lets you know in a coy rather than charmless fashion, which just makes it all the more irresistible. Forget having your name in lights. How about having your name in these particular sans-serif capitals?

It’s also a typeface to fall in love under.  A first swoon beneath this kind of sign? There’s little that could be more provincially sentimental.

Facts, rather than fancy, reveal that as with most exotica, the logo blazed brightly for a period, then fell out of favour sharply. Thankfully it gave way not to something worse, but something even better. Father and son eye each other warily outside Russell Square, positioned at right angles in a way that flatters neither. But then neither belongs next to the other. It’s like putting two Dr Whos in the same show and expecting double the fun.

Facts also reveal that the typeface does indeed date from before the war – but the first world war, not the second, and therefore more of a veteran than its slightly futurist style suggests. It’s as old as 1908, the year the Underground got tired of the 20th century breathing down its neck and decided it needed a trademark. But the network ended up not only conceiving its own logo but its own emotion. For here was where the romance of the Underground first spluttered into existence – a romance that continues to this day.