Different film formats
Negatives can appear on many different types of film and formats. You can find a handy overview here.
Until the 1930s, film was made from nitrate, a highly flammable material that releases nitric acid as it chemically decomposes. Nitrate film was then replaced by cellulose acetate film, but this gave rise to a new problem because this carrier is susceptible to vinegar syndrome. If the film smells of vinegar, or you see any cracks and the emulsion layer coming away from the carrier, these are signs of vinegar syndrome.
Negatives on nitrate or acetate film should therefore be digitised as soon as possible.
You can find a detailed guide at projectcest.be that will help you to identify nitrate film. Filmcare can also help with identifying nitrate, cellulose acetate or polyester film.