Spring and Autumn models are generally 0-4 months. It is recommended that the inner length be 10.5cm... 4-6 months. It is recommended that the inner length be 11cm... 6-8 months. It is recommended that the inner length be 11.5cm... 9-10 months. It is recommended that the inner length be 12cm-more than 10 months depends on the baby's foot length (each baby has different development, try to measure the foot length before choosing). Spring and Autumn recommended foot length + 0.5cm = inner length slightly fat recommended + 1cm equal to the inner length of shoes