A stretchable carrier that has been provided with four straight stretcher bars that are divided in the corners by surfaces that stretch along the bisecting line of the corner between the center line of the stretcher bars that meet in the corner and at least one key fixed in a corner that is connected with both stretcher bars meeting in the said corner by means of friction effected by being jammed between the stretcher bars, whereby the top angle of the key is aimed at the corner and whereby the angle of the key is larger than the angle enclosed by the center lines of the stretcher bars meeting in the said corner. As this key is connected to the stretcher bars by this jamming and friction, the stretcher bars can be adjusted to any required dimensions without problems when connecting the stretcher bars.