Once children have developed their fine motor skills, worked on learning letters and practiced writing on blank paper and with other mediums to develop their letter formation, they are usually introduced to handwriting paper with dotted-thirds to assist them in writing letters of consistent size and format.
A little trick I used in my classroom to help children learning to write in dotted-thirds was, "Clever Cat."
Clever Cat's bottom sits on the solid base line and his body takes up the first gap. His head sits on top, occupying the second gap up between the dotted lines and his tail dangles below the solid line, extending to the first dotted line below.
Clever Cat for handwriting works as a fun visual aid to assist children in remembering where letters should sit between the lines and their sizing.