A device for positioning and joining at an angle, by means of a sewing machine, the borders of two flaps of fabric, particularly of a horizontal cloth and of a vertical band for forming the case of a mattress, having an angular element which comprises a first laminar channel, forming a guide for the border of the cloth, and a second laminar channel, which forms a guide for the border of the band; the channels are arranged at a right angle and each channel is comprised between two walls: one wall is external to the channels and the other wall is internal thereto; respective through openings are formed in the outer wall and two feeders are also provided which comprise rotating heads which are arranged so as to tangentially engage the cloth and the band through the through openings in order to place the borders of the cloth and of the band, guided in the channels, adjacent to each other.