Abstract: A guide for aligning pickets during the building of a picket fence includes a beam having a lower straightedge against which the tops of the pickets may be abutted during installation. The beam has a length approximately that of the typical distance between adjacent fence posts. The "picket end" of the beam is supported on the top of the last placed picket, with the beam face resting on the picket top, and front and rear legs at the picket end straddle the placed picket and the top rail and are spaced apart a distance equivalent to the thickness of the rail and picket to stabilize that end in the proper supported position. At the "rail end" of the beam, stabilizing legs are spaced apart a distance equivalent to the thickness of the rail; and a spacer block adjustably mounted between these legs rests on the top rail and establishes the rise of the beam at this rail end of the guide.