Imagine, if you will, time being like the letter "Y". You live in the top right-hand bit; Jemima and Patricia live in the top left-hand bit.
If you were to be fortunate enough to be in possession of a time machine, you could go backwards to see what happened in the long straight bit of the letter "Y". If Jemima and Patricia had the equal good fortune to have such a device they could also go backwards to find out what the olden days were really like. If, say they developed a rather improbable fascination with the American Revolution they could pop back to 1776 and watch John Hancock put his over-the-top signature to the Declaration of Independence. If you took your time machine back to the same year, you could wave at Patricia and her friend (before being ejected as undesirables by the wealthy gentlemen signing the Declaration).
If the girls' new extraordinary passion for history were to hold, they might want to go forwards and see the look on MP William Wilberforce's face as the Slave Trade that he had campaigned against all his life was abolished by Britain's parliament in 1807. If you were to choose the same year, you could wave at the girls again: fancy bumping into you a second time in this vast vortex, you might say (before being ejected by the wealthy gentlemen who make Britain's laws).
Then the girls might want to go forwards again and see what happened in 1832.
If you were to arrive in that same year, the girls would start to look a bit see-through to you, just as you would to them. For this year is the top of the stalk of that letter "Y" I've been telling you about. History branches this year. From your point of view this is not the most exciting or well-known period of your history. You may not even be sure why you're here. However, if you're a keen historian you'll know that something with far-reaching consequences happened this year, thanks to Earl Grey. Yes, it's amazing the far-reaching consequences of a cup of tea, isn't it? No, not that kind of Earl Grey: the Prime Minister, Earl Grey. He was a Whig Prime Minister who managed to bring in the Representation of the People Act. Within years of this you're looking at a new Britain: the monarch's been reduced to a mere figurehead; the amount of people eligible to vote has greatly increased; body-snatching has virtually disappeared; children are no longer being hanged for their theft of a hankerchief worth more than a shilling; slavery has been abolished. Within a few decades the Whigs and the Tories have become the Liberals and the Conservatives - political parties which would have been unrecognisable back in the 1830s. This new Britain will evolve and evolve into the one we know today.
However, you won't meet Patricia and Jemima there. No, they live on the parallel branch of the letter "Y". There was no Representation of the People Act in 1832 for them, which is why their history since has been so different.
If I can bring you forwards to their 1860s, President Jefferson Davis won the Southern War of Independence and the Confederate States of America gained their freedom to carry on denying slaves their freedom. They seceded from the United States of America thanks to the interference of the French, the Russians and, of course, the British, all of whom wanted to see the USA split into two unthreatening halves.
The Reform Bills of 1867 and beyond never happened in Britain.
New Zealand, being part of the British Empire, did not grant votes to the Maoris and the NZ women in 1893.
Three of Queen Victoria's grandsons (the German Kaiser, the Russian Czar and the British King George V) got together and sorted their differences out in the early 20th century. Result: no First World War.
Without this the Kaiser and his descendants continued to run Germany while the Czar and his descendants held onto Russia..
So no Nazism or Communism, then. But Fascism? Oh, yes: Italy still went Fascist but, unlike in our parallel world, they stayed Fascist.
This means the only true democracy in Jemima's world today is the United States of America, which, if you remember, is comparatively small.
Britain certainly isn't a true democracy: the Queen has a lot of genuine say, as do the House of Lords, and almost nobody has a vote for the one pseudo-democratic element of the whole thing, i.e. the House of Commons. You'll see the same two parties who've been around there for centuries, i.e. the Whigs and the Tories, both utterly unreformed and keen smokers, drinkers and gamblers. There is no secret ballot and votes can - quite legally - be bought for the right price (from the handful of people allowed a vote). There are "rotten boroughs" where the amount of voters is frequently one to ten people.
However, for someone like a Jemima or a Patricia, the particular year you live in - and the particular branch of the letter "Y" - is something you have no real awareness of. You get your mobile 'phone out, chat to your mates, eat enough to bring a grateful tear to the eye of any doughnut-salesman, worry about whether you've done enough washing to last for the next few days and you wonder whether to take the boy opposite you up on the offer he's just made you...