1a hair has almost never heard of frizz and hangs like a silky sheet. Even when it's thick, folks with 1a hair tend to think or be told that their hair looks thin when it's loose because it generally has no volume to it and the strands hang very close together, since they have no back-and-forth to hold them separate.
1b tries to do that, but has just a little bit of minor wave to it, maybe the ends flick this way or that, that sort of thing. It's still definitely straight hair (of course, I can't tell you how many 2s and 3s start off thinking they're straighties with frizz, so that might not be an obvious description), but it isn't mirror-smooth straight.
1c definitely is heading toward the wavy category, even though it's mostly straight. It tends to have a bit of frizz where individual hairs want to pop out of the pattern, and it often dips in at the neck, out by the shoulders, and maybe in and out once or twice more as it gets long. It might have an S curve near the face. Without having studied it, I'd guesstimate that 1c is particularly common among straightish-haired folks of european descent, more so than 1a or even 1b.
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