PECS:

The Picture Exchange Communication System or PECS, allows people with little or no communication abilities to communicate using pictures.

The Six Phases of the Picture Exchange Communication System Are: 

PECS PHASE I: How to Communicate
The child with autism learns to exchange single pictures for items or activities they really want.

PECS PHASE II: Distance and Persistence 
Still using single pictures, the child with autism learn to generalize this new skill by using it in different places, with different people and across distances. They are also taught to be more persistent communicators.

PECS PHASE III: Picture Discrimination 
The child with autism learns to select from two or more pictures to ask for their favorite things. These are placed in a communication book, a ring binder with Velcro strips where pictures are stored and easily removed for communication.

PECS PHASE IV: Sentence Structure
The child with autism learns to construct simple sentences on a detachable sentence strip using an "I want" picture followed by a picture of the item being requested.

PECS PHASE V: Answering Questions 
The child with autism learns to use PECS to answer the question, "What do you want?"

PECS PHASE VI: Commenting 
Now the child with autism is taught to comment in response to questions such as, What do you see?, What do you hear? and What is it? They learn to make up sentences starting with I see, I hear, I feel, It is a, etc.