When scanning archives from the 1960s and 1970s, you may encounter 126 format "Instamatic" slides.
Most film scanners have sensors designed for the 36x24mm image area of conventional 35mm film (more formally known as 135 format). The image area for 126 film is 28x28mm, which is 4mm taller than most scanners can see. A flatbed scanner with good film support can get everything, but these are generally less effective than a dedicated film scanner.
I suspect that most people just satisfy themselves with the central 28x24mm portion of the Instamatic image, losing a 2mm band on the top and bottom. Most photos are framed so that the important details are in the center, but I use this procedure to capture the whole thing.
This only works for slides. You'll have to settle for a flatbed scanner if you have 126 negative strips.
This technique assumes that you have a scanner with a separate slide holder. This works well with my Primefilm-Reflecta 7200, and should work with Plustek or other scanners that uses a similar holder.
There is usually some slack so that you can shift the slide a bit, which lets us scan the whole slide in two passes, then manually merge the results.
Start with a normal 35mm slide. Insert it in the holder and then mark guides on each side of the holder at the edge of the image area. We know that the sensor in the scanner can see at least to this point.
Next, use these guides when inserting a 126 slide. If you insert the 126 slide fully, the top 2mm of the image will be out of range, so shift the slide down slightly so that the top edge of the film aligns with your guides.
Scan the slide to capture the top-most part of the image, then remove, rotate 180° and repeat the process to scan the bottom-most part.
Sew both images together in Photoshop and crop. The details below discuss how to keep the scanning parameters consistent between passes so that it is easier to merge the images.
In order to maintain consistency between scans, I follow the steps below. These are described in terms of VueScan, but there should be similar capabilities in whatever you are using to drive your scanner.