Folding knives have been around since the Roman empire, and the Vikings had some beautiful ones. A great looking Roman era folding knife is shown here.
Pocket knives were popular as a picnic item, with folding forks and spoons. They especially became popular when pockets became popular in Europe. They are also popular for trimming quill ink pens, from which the alternative name "Pen knife" arose. Knives which used a spring on the back to hold a blade in the closed and open position came into use at least by the 16th century. Barlow Knives were popular in early America, and continue to be made. Lock back knives were perfected around the turn of the 20th century, and many models were available.
The diagram below shows the parts of a typical spring back pocket knife. This combination of blades is called a Scout model, and many Official Boy Scout knives are made in this style.