Whilst I would never claim that my Darkisle novels should be classified as Historic Fiction, the fact remains that they are set in our world, during the 1920s. Very few people still living will remember that period, but all the same it’s a time we all know SOMETHING about . That means the reader has every right to expect that the background of the stories feels authentically Twenties-ish. Jack, my protagonist, shouldn’t be able to hop on a jet airliner, or cross the Channel on a hovercraft, or flash a credit card, or even make a direct telephone call to someone. Should a reader encounter something they know is wrong for the period, it will jar. And that jarring will take them out of the place that I, as the writer, want to transport them to.
Okay, it could be argued that getting the FEEL of the period is more important than getting every single fact correct. Does it matter if Jack made a telephone call without going through the operator, so long as the conversation he then had used the right words for the time and moved the story along? For many readers, perhaps not. But for some, it will. I see it as my job to keep jarring to an absolute minimum – and the only way to do that is to check the facts. I suppose, if I had wanted an easier life, I’d be writing either contemporary fiction (where I already know the technology and language in use) or fantasy (where they’re what I damn well say they are!).
I can think of a couple of examples. In ‘Tribute’, the middle story in my ‘Hobbs Top & Other Horrors’ collection, I wanted a ten shilling note to pass hands. A nice old-fashioned touch, I thought, as the note was phased out in the 1960s. But something made me check – and lo, the ten shilling note wasn’t issued until 1928. And ‘Tribute’ is set in 1923! So, back I went and changed the text to ‘two crowns’ – which was the same amount of money, just without the flourish of waving a piece of paper. Would leaving a five-year-too-early banknote in the story have been spotted by many readers? Probably not. But it was wrong and I knew it was wrong, so it had to go!
Something similar happened when looking for a disparaging word for a policeman that Jack could use. ‘Plod’ sounded perfect… until a quick check revealed that its use was inspired by the character ‘PC Plod’ in an Enid Blyton book that wasn’t published until 1949. Darn it!
Luckily, I’ve found a great resource to help me. It’s Ngrams, an application in Google Books that has values for the frequency of use of words in a number of languages over several centuries (and, for English, offers either British English, American English or both combined). It really is a boon!
Let me give you an example. In ‘Those Under The Hill’, Jack is lent a bag to use when out walking. But what to call it? Now, to my modern mind if I have a bag with straps that I put my arms through so I can carry it on my back, I call it a backpack. But that has a modern ring to it, so I checked it on Ngrams, which returned the following:

Uh oh. ‘Backpack’ wasn’t used at all until the 1970s and really only took off in popularity this century (as you can see, Ngrams gives different returns when you search on a single word, depending on case. Not sure if I like this feature or not). Ok, my second choice word was ‘rucksack’, so I checked that out next. A useful feature of Ngrams is that it lets you compare multiple words. So, if you’re torn between two or three choices, you can see which would be most likely used at a given date. Here’s the result for both ‘backpack’ and ‘rucksack’.

Ok, so for my novel set in 1922, ‘rucksack’ looks alright. Its usage line twitches into life just after 1900 and the word was slowly increasing in popularity by the 1920s. Hmm, but then there’s ‘knapsack’ – that’s quite an old-fashioned word, isn’t it? I ran a comparison of that with the other two words.

Right! So, while people aren’t all that likely to say ‘knapsack’ nowadays, back in the Twenties it was the go-to word. Armed with that knowledge, Jack takes his trusty Webley revolver and a map, throws them in a knapsack and heads for the hills.
Here’s another example; should a novel set in the past use the word ‘jail’ or ‘gaol’? Well, Ngrams can help with that:

As you can see, in the England of 1922 the ‘gaol’ spelling would have been preferred – but ‘jail’ was by no means unheard of, so I can get away with it if I want. Incidentally, this is often seen as a case of an Americanism pushing out an older British spelling; not true. Both spellings have co-existed since they were coined in Middle English. What is true is that the eventual preference for ‘jail’ in the USA sparked the decline in ‘gaol’ elsewhere.
I should also point out that words that you think are dead and buried often seem to be used more frequently nowadays than you might expect – this could be down to the number of articles on the internet saying something like “gosh, nobody says ‘knapsack’ or ‘gaol’ anymore”.
Finally, I should just mention that Ngrams isn’t limited to single words. Here’s how it would have helped with my ‘PC Plod’ research:

So, there you have it! I think Ngrams is a pretty cool resource if you want to set a story in the past. I’m not saying my 1920s characters will now never utter a word or phrase from a later era (or an earlier one – that can happen at times!). But at least the chances of it happening are a good deal less now I know about Ngrams.
Why not have a play for yourself? Have a look at https://books.google.com/ngrams.