I don't want to complicate this discussion much further, but don't know any other way to add a few comments.
There is so such thing as a "12 volt" coil. Just coils designed for certain "12 volt systems." If a coil cannot fire at 9 volts, it's going to be useless in a 12 volt system.
One thing that is universal is . . . battery voltage normally drops to 9 volts when the starter motor is cranking in cold weather. So, if you want good spark AT that time, you need a coil that can do it on 9 volts, NOT 12 volts.
For this reason, the vast majority of cars and trucks that had breaker-point ignition and 12 volt systems used what many call "6 volt" coils. They are often the exact same coils as had been used previously in 6 volt systems - with a resistor added.
You do NOT have to use a resistor and bypass system. If you do, you can use a stand-alone relay with a built-in bypass, or a starter-attached solenoid with a bypass (e.g. Delco), or a key switch with bypass (like Deere used on many 10 series tractors). Some tractors even used a separate "cold start bypass" switch.
Some resistors are made to be very heat and amp-draw reactive e.g. like some Ford tractors use. The resistor adjusts itself and allows full voltage when first cranking, but once the engine is running and the resistor gets hot, it cuts voltage way back.
It makes things easier to refer to coils by their primary resistance - rather then "6 volt" or "12 volt."
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