Stretcher bars and strainer bars are not the same. Both are auxiliary supports for paintings but one has rigid corners and the other has expandable corners. A stretcher bar is expandable. Wooden keys are used in the corners to either increase or decrease the tension on the artwork:
 
A strainer has non-expandable corners and does not allow for increasing or decreasing the tension of the artwork once it is attached.
 Rigid corners
Framing Matters recommends stretcher bars to be used for framing valuable artworks.
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