A substantial increase in maxillary width is usually best
obtained by placing:
In rapid maxillary expansion appliance, when a rapid heavy force is applied to the teeth, there would not be enough time for tooth movement to take place; hence, the force is directly transmitted to the sutures. 
The appliance compresses the periodontal ligament thereby bending the alveolar process, tipping the anchor teeth, and gradually opening the midpalatal suture. 
The sutures open with minimal movement of the teeth.
There is substantial increase in the maxillary width with fixed appliance than removable appliance. 

​Textbook of ORTHODONTICS Sridhar Premkumar
Lingual arch wire
A sutural expansion fixed appliance
Posterior intermaxillary cross elastics
A face bow with an expanded inner bow