A direct retainer is any unit of a removable dental prosthesis that engages an abutment tooth to resist displacement of the prosthesis away from basal seat tissue.
The direct retainer's ability to resist this movement is greatly influenced by the stability and support of the prosthesis provided by major and minor connectors, rests, and tissue bases.
The intracoronal retainer is either cast or attached totally within the restored natural contours of an abutment tooth. It is typically composed of a prefabricated key and keyway , with opposing vertical parallel walls, which serve to limit movement and resist removal of the partial denture through frictional resistance . The intracoronal retainer is usually regarded as an internal or a precision attachment. Keyways are contained within abutment crowns, and keys are attached to removable partial denture framework. It is used as retainer for tooth-born situation, where vertical movement only is allowed, or for joining two parts of fixed partial denture.

It refers to an interlocking device, one component of which is fixed to an abutment or abutments, while the other is integrated into a removable prosthesis to stabilize and/or retain it.
Internal unit: attached to the framework.
Receptacle: as vertical wall within the abutment.
Improve esthetic by elimination of the visible retentive and support components.
Improve patient comfort.
Improve chewing efficiency.
Better force distribution on abutment teeth.
Reduction of food impaction.
It requires preparation of the abutment tooth and casting crown.
It requires complicated clinical and laboratory work.
It loses its retention with loss of frictional resistance.
Difficult to repair and replace.
Least effective in short teeth.
Difficult to place completely within the circumference of abutment tooth.
More costly.
Cannot be used with true removable partial denture.

Intracoronal: are incorporated entirely within the contour of the crown, the attachment female to be incorporated into a crown contour. Its advantages that the occlusal forces exerted upon the abutment tooth are applied close to the long axis of the tooth.
Extracoronal: are positioned entirely outside the crown contour. The advantages include that the normal tooth contour is maintained, minimal tooth reduction is required and the possibility of devitalizing the tooth is reduced. Also the path of insertion is easier for the patient.
Pulp size: should be protected and or jeopardized by the attachment component.
Length of the clinical crown: this may affect the amount of frictional resistance for retention.
Cost of treatment.
These attachments might be classified on the type of restoration, they are designed for: removable partial denture, overdenture, implant or fixed prosthesis .