Miss Sullivan had been trained at the Perkins Institute for the Blind, where she had gone because a bacterial infection had left her almost blind. At the institute, eye operations improved her sight a good deal. She also learned the manual alphabet, a way of writing into another person's palm.
The key to Miss Sullivan's ability to teach Helen despite her pupil's blindness and deafness was the manual writing. Miss Sullivan could write in Helen's hand, and Helen could comprehend what her teacher was communicating through touch.
The biggest obstacle Miss Sullivan faced was getting her young charge to understand that what she was writing in her hand had meaning. She needed Helen to connect the writing to objects in her world. Once this was accomplished, Miss Sullivan had a reliable method to teach her student.