Pediatrician - After 18m check-up whats next: 21m checkup or 2yr checkup?

AB

Was at the pediatrician last week with my newborn and 18m old for back to back appointments. On my way out I scheduled my newborns next one but forgot to schedule my 18m olds. What’s the next appointment - is it in 3m when he’s 21m old or in 6m when he’s 2?

Of course Ill call tomorrow and schedule either way, but now I’m just curious what’s the next one?

Thanks