The main aim of this article is to determine a suitable proportion and shape for monitor skylights, which are characterized mainly by two opposite vertical openings fixed by a reflector of variable shape, for ensuring maximum illuminance on the work plane within a room. This type of skylight has a reflector, which is usually rectangular, slanted, sawtooth or curved, behind the vertical openings. Lightscape 3.2 software was used to carry out the simulations of different rooms with four different skylight shapes. All trials are carried out under overcast sky conditions, considered the worst case scenario. Following the trials it is concluded that for this type of skylight, the highest average daylight factors on the work plane are obtained for a height/width ratio close to 1/1, regardless of the reflector shape. In addition, it is concluded that the performance of the rectangular shape is poorer than that of the other shapes; the slanted shape has a better performance when the height/width ratio is lower, the curved shape produces greater daylight when the height/width ratio tends toward 1/1 and the sawtooth shape generates higher daylight factors when the height/width ratio is higher. (C) 2014 Elsevier B.V. All rights reserved.