The apertures of the PORTRAIT medium format mounts are 40 mm x 50 mm (width by height). These are appropriate for 6 x 4.5 format slides, or cropping 6 x 6 slides.
These mounts are the same design as the cardboard mounts previously sold by Kiewa Valley in Australia and by Rocky Mountain Memories in the USA. These used to be called Alpers or Spicer mounts. The apertures are spaced 62mm apart, making them suitable for viewers with an interocular distance of approximately 65mm (or larger). The overall dimension of the folded mount is 132mm x 80mm. The rear apertures are a bit larger from the front apertures such that the stereo window is always defined by the front apertures, even if there is a slight error in folding. The corners of the front apertures are rounded. The cardstock is heavy weight (110lbs), black in colour, and is advertized as acid and lignin free. The mounts are creased for easy folding, but will be shipped unfolded.