Dating Old Photographs
In your collection of old photos, do you have some with no dates nor any clear way of telling when the photo was taken? If you know who the person is, then you can narrow down the dates somewhat but who doesn't have a few mystery photos with no notes sitting in their history collections, just crying out for further identification.
If you can pinpoint the date (somewhat) then you may even be able to figure out who the person is by connecting them with the right generation in your family tree. Family resemblance is a very handy tool, but remember that your great-grandmother may have looked an awful lot like your own mother when she was younger. Dates can help eliminate that problem.
The most common way to date photos is by fashion styles, especially in the mid- to late- 1800s when nearly all photos were taken at professional studios, and had no background items to help. Unfortunately, with fashion being the changeable creature it is, this is not a very reliable way to date photos. Some fashion trends lasted for several decades, and you can never tell how current a person's clothes are from a photo. But it is better than nothing.
Ladies fashions are usually used as a date indicator, since they changed far more frequently than men's fashion. Dress styles, presence or absence of hats and other accessories, hair styles and even poses. Fashion-Era is an excellent website to explore the fashion styles of the past. There are pages for the 1800s but most of the site focuses on the styles of the 1900s. You can also find some great examples and quick things to look for at Family Chronicle.
Another way to tell the age of the photo, is the style and format of the photo itself. Is it printed on a cardboard card with the photographer's name on the back, or is it a thin sheet of metal mounted in a matte frame? Or are your family photos actually printed as postcards, designed for easy mailing? You can narrow down the date window by using the type of photo.
The Brownie camera was invented in 1900 and became wildly popular in the 1950s, allowing anyone to take their own photographs. During this time period, you'll find many more amateur photos which can offer many more clues by letting you see the backgrounds of candid shots. Once cameras were basically available to anyone, you'll find many more family photos since it no longer required an expensive trip to the studio.
Modern-era photos are much easier to identify because we are all still reasonably familiar with the time frame, including fashion and other indicators. That photo of your aunt in the day-glo bell bottoms must certainly date to the 70s.