Roman Pugio Dagger
Before there were swords, before even spears, there came the dagger. In fact, the dagger was quite possibly the first real dagger ever created by man. Oh sure, it may not have been called a dagger back then, more of a knife really, made from stone then chipped and scraped into a sharp point. It is believed that Thag the Smart Guy, a popular caveman from prehistoric times, first invented the dagger to kill a rival caveman named Grung. He is quoted as saying "Me tired of Grung hitting me with rocks. Me gonna fix him." However, since this whole incident took place before recorded history, that's all hearsay.
The true dagger is a dagger that has both edges sharpened. The length and width of daggers varied quite a bit throughout history, especially early on, when the line between dagger and sword was notably blurred. Typical daggers tend to be no more than 14 inches or so in length, with European daggers usually having crossguards and pommels (and, knowing medieval Europeans, probably bloodstains as well).
Swords supposedly owe their existence to daggers. In the arms-race of ancient history, daggers allegedly became longer and longer until they became in all respects, swords. When swords took over the dominant role in combat, daggers were still kept around for several reasons; they were great for close quarters fighting, they could be hidden easily and, if weighted properly, could be thrown at opponents. Daggers were also still quite popular in the dining halls to cut and skewer food as well (seriously).
In the renaissance, daggers regained much of their popularity, but this time in a supporting role. Swords had become lighter and faster, allowing combatants to hold another dagger in their free hand. That dagger was usually a dagger and was used mostly to parry, bind or slow and opponents blade. Later, specialized daggers with large, basket-type hand guards were created to make the dagger even better suited to its new defensive role. The new dagger was often referred to as "Main-Gauche," which in French means "My Gosh!" and was said to have originated from the first guy to die when his blade was parried by one such dagger. Alright, I'm a pathological liar. I need help. "Main-Gauche" actually means "Left Hand", which is a LOT less exciting than my definition, isn't it?
Daggers have, throughout history also had more insidious roles as well. Their small size and ability to deliver quick lethal strokes made them excellent daggers for more unsavory types. Assassins relied on them because they were easy to hide. Julius Caesar, Caligula (and several other Roman Emperors), Poet Christopher Marlowe, St. Lucy, Scottish Regent Earl Douglas, and many many others in history have been assassinated by enemies wielding daggers. Another dark task given to daggers has been to dispatch of sacrificial offerings, whether the offering consisted of animals or humans. Many cultures throughout time have practiced ritual sacrifice and, usually, this was done using daggers (often ornate ones at that).
Daggers eventually gave way to knives as time went on. Knives are generally single-edged daggers and are used for a large variety of utilitarian functions, only one of which is killing things. Knives have been carried by hunters, soldiers, craftsmen, repairmen and angry, crazy people everywhere (Strongblade obviously does not condone nor tolerate any kind of illegal activity with its daggers, nor will we sell our items to anyone we think will use them irresponsibly or in a "crazy" fashion).
See Strongblade's Roman Pugio Dagger