I have a bad memory, photographs help me remember.

A half-frame camera is a camera using a film format at half the intended exposure format. A common variety is the 18x24mm format on regular 135 film. It is the normal exposure format on 35mm movie cameras. For still cameras using the 35mm film, the usual format is 24x36mm, so still cameras taking 18x24mm exposures are called half-frame cameras.

There was a vogue of half-frame cameras in the 1960s, mainly from Japan, originating with the Olympus Pen models. It allowed to build a very compact camera, yet using the regular film that can be bought anywhere, unlike the other subminiatures that used exotic films (16mm, 9.5mm, etc.). This vogue ended when cameras like the Rollei 35 or the Olympus XA showed that it was possible to make cameras as small as the half-frame ones, but taking full 24x36mm exposures.

With a half-frame camera, one can fit twice as many pictures onto a standard roll of film. For example, 72 exposures on a 36-exposure roll, 48 on a 24-exposure one, and so on.

The most advanced half-frame camera that was designed as such from the start is the Olympus Pen F single lens reflex.

For some specific needs, there were cameras originally designed for full-frame pictures that were produced in very small series as half-frame models, for example some Leica, Nikon or Robot rangefinders, and some Alpa SLRs. These are mainly interesting as collectibles.
– from wikipedia.

this is what a half frame film looks like

this is what a full frame film looks like

this is inside the View finder of Canon Demi EE17

I give 2 stars to this camera..
its not a very good camera, but it looks cool
its not as small as it looks, its heavy too.