Baby sizes help please

I'm a idiot..lol. Idk why but I can't figure out what size to buy my daughter for this summer...

Shes 11 months and she currently wear 9-12 and 12 month clothes. She turns 1 next month would I get her 12 months for summer or 18months...

I'm just confused is "12 months" 12 through 18 months and then 18 months through 24 or is 18 months actually for 12 months to 18...I've completely confused myself to say the least. Lol 😂🙈