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There are basically two types of partial dentures; all acrylic and those with a metal framework. For strength and longevity, the metal framework type partials are the best. The metal used is a chrome-cobalt based material which allows the partial to be extremely thin but strong. Acrylic plates are thicker and give the patient the feeling that there is not as much room in their mouths for the tongue, therefore affecting the speech more.
The following link has more information about partial dentures along with some photographs:
I hope this answers your question. If you would like to discuss this further or have any additional questions, please reply to my answer and I will get right back to you.
Sincerely, XXXXX XXXXX DDS