You're not from around here, are you?
A coworker once mentioned – twenty years ago in Spokane, the biggest small town in Eastern Washington – that he'd noticed me driving eastbound on I-90 while he was driving westbound over the weekend. He wondered if I'd gone for a hike in Idaho.
I was flabbergasted. How could you "notice" a person you know driving the other direction on a multi-lane interstate highway? Did he study every one of the thousands of cars he passed while driving, scanning for people he knew? I was driving a nondescript white SUV at the time, a common vehicle in Spokane.
I realized later that the difference in our perspectives was due to the fact I was a city boy and he was a country boy. I had moved to Spokane that year after 20 years in Chicago, where nobody pays any attention to the flood of vehicles going the other direction on the Kennedy, Dan Ryan, or Edens expressways every day. But my coworker had recently moved to Spokane from eastern Montana, where you can drive for miles without seeing another vehicle. And when you do see somebody coming, there's a chance you might know them, because why the hell would anyone not from around here be driving down this country road?
In addition to checking out the people in approaching cars, another small-town habit is checking the license plate. If it's an out of state plate, they're not from around here and different rules apply. Last year, I was waving at every vehicle I passed on a country road, and a big guy in an old pickup trucked waved back with an obscene gesture. I mentioned this to a local friend, who replied "oh, that's just the out of state plates, it will be fine after you get Montana plates."
I picked up the habit of checking out license plates after moving to Butte, Montana, and I soon noticed that most Montana plates start with a 1 or 2 digit prefix such as 4-, 6-, or 30-. Some prefixes are more common than others. 51- is pretty common here in Butte, but the one you see by far the most often is 1-. I have a 1- prefix on the Montana plates on my truck, which I picked up at the county courthouse last summer.
It turns out that every one of Montana's 56 counties has an assigned prefix for license plates in that county, and Butte-Silver Bow's prefix is 1-. Here's the official list of prefixes from the Montana state government web site:
How did these prefixes get assigned, and how did 1- get assigned to Butte? The common answer you'll hear is that the prefixes were assigned based on the ranking of county populations in the 1930 census, but that's not actually true for many counties. It's true that Butte and Silver Bow County were still Montana's biggest population center in 1930, so Butte's 1- makes sense, but others on the list don't fit the facts.
For example, sparsely populated Powder River County has a 9- prefix, but in the 1930 census its population was 46th largest in the state, so it should have had a 46- prefix. And Lincoln County was assigned the final 56- prefix, but they should have a received a 27- prefix as the 27th largest county in the census. The Billings Gazette did a nice article entitled What's the deal with Montana's license plate numbering system? a few years ago showing how the prefixes compare to the realities of the 1930 census, and what the prefixes would be if assigned based on the most recent census. I've not seen a clear explanation of these discrepancies, but considering Montana's political history, one assumes smoke-filled rooms and arm-twisting or pocket-stuffing were involved.
These prefixes are only used on standard plates, and other types of license plates don't have a prefix. For example, when a county has a custom plate that identifies the county, then they use an alpha prefix. And there are reserved alpha prefixes for a bunch of other things, too: disabilities, WW II vets, National Guard people, etc.
Once you understand the prefixes and have the ones in your area memorized, you can start to think in terms of "people from around here" (same prefix), "people from nearby," and "people from the far corners of Montana."