We have investigated the vertical growth of citrate-free Ag nanoplates into truncated right bipyramids and twinned cubes with truncated corners in the presence of Cl- ions at low and high concentrations, respectively, with Au serving as a marker for electron microscopy analysis. Both the Cl- ions and Au atoms could be introduced through the use of HAuCl4 as a dual agent. When HAuCl4 was added into an aqueous mixture of citrate-free Ag nanoplates, ascorbic acid (AA), and poly(vinylpyrrolidone), Au would be immediately formed and deposited on the surfaces of the nanoplates due to the reduction by both Ag and AA. The deposited Au could be easily resolved under STEM to reveal the growth patterns of the nanoplates. We found that the presence of Au did not change the growth pattern of the original Ag nanoplates. In contrast, the Cl- ions could deterministically direct the formation of Ag nanoplates with a triangular or hexagonal shape, followed by their further growth into truncated right bipyramids or twinned cubes with truncated corners upon the introduction of AgNO3. This work demonstrates, for the first time, that citrate-free Ag nanoplates could be transformed into right bipyramids or twinned cubes by controlling a single experimental parameter: the concentration of Cl- ions in the growth solution. The mechanistic understanding represents a step forward toward the rational design and shape-controlled synthesis of nanocrystals with desired properties.