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First, some background: Developer works by turning silver halide crystals on the film base which have been exposed to light into metallic silver. When the film is put into fixer, silver halide (unexposed) is dissolved and any metallic silver (exposed) is left alone. Being an analog process, crystals which have been exposed to more light will react with the developer more rapidly than those exposed to less. There are other factors that have an impact on the reaction rate, notably the temperature and how much the solution is agitated, but let's assume they remain constant.

If you don't leave your film in the developer long enough, less of the halide crystals will turn to silver, and more of them will be dissolved during fixing. The result will be a "thin" negative that's mostly see-through. An extreme version of this would be to process a piece of scrap film in fixer without developing it first. Since none of the silver halides have become metallic, all of them will be removed and you'll be left with just the film base.

Leaving the film in the developer longer gives the less-exposed crystals more opportunity to react, resulting in more metallic silver being left on the base that won't dissolve during fixing. The resulting negative will be "thick" and harder to see through. Leave the film in the developer long enough and you'll get black negatives when you pull them from the fixer.

In both cases, one clear sign of over- or under-development is a lack of detail in the negative, because the details tend not to be solidly exposed or not exposed. Being on the wrong side of "correct" development will mean those parts react and swept away in the fix or don't and remain on the base.

If you've heard the terms "push" or "pull" processing, they mean the deliberate use of overdevelopment to bring an image out of underexposed film and deliberate underdevelopment to prevent overexposed film from going black. Most of us do this to some degree with paper prints under safelight by watching for a satisfactory level of development and putting it into stop bath to halt the process.

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Just a note: A negative that is visibly thin (or visibly blocked) will tell you when the neg is severely out-of-range. Being just a little off shows up in contrast. Since the deep shadow areas develop as far as they're going to develop very quickly, the time in the bath mostly affects the highlights. Too little time, and the negative will be flat; too much time and it will be too contrasty. You can make up for that by changing the paper contrast (and you pretty much have to with roll film, since exposing and developing each shot on the roll for optimum contrast is nearly impossible).