They are not all twin size. We are simply using twin size mattresses as an example, so we are not comparing different sized mattresses. As made clear earlier, the term ‘twin size’ refers to two twinned single beds being used in the same bedroom. Standard bed sizes in the US and Canada are: Twin or single. 75 in × 38 in (191 cm × 97 cm). They are ‘twins’ because they are the same size and design, and intended for use by two children in the same room (one in each bed). The smallest in our review is 74” x 38” and the largest 75” x 39”. So there is only one inch difference either way.