Diagonal, curvy and straight, no matter how you make lines, you're getting your child ready to write letters!
The next time your child draws, talk about shapes and how letters are made up of shapes. "Look, that triangle almost looks like the letter A, all you need are two straight lines at the bottom! To make the letter O, you draw a circle."
Describe the direction and shape of your child's lines, too. "I see you're making a diagonal line. Can you draw a curvy line?"
Direction and shapes will help your child learn how to read and write when they reach kindergarten.