Ewart Oakeshott (1916-2002) was a lifelong enthusiast, scholar, and collector of medieval swords, and his publications are standard English-language reference materials for many collectors and recreationists. He was a Fellow of the Society of Antiquaries, a Founder Member of the Arms and Armour Society and the Founder of the Oakeshott Institute.
His first book, The Archeology of Weapons: Arms and Armour from Prehistory to the Age of Chivalry established the system of medieval sword classification broadly used today.
Oakeshott Sword Typology
Building on the Norse sword classification typology developed by Jan Petersen and simplified by R.E.M. Wheeler, the Oakeshott classification system defines two main groups totalling thirteen distinct blade types labelled X through XXII, with subgroups within a type noted by an additional letter (i.e. Type XIIIa).
In addition, he codified 25 separate types of pommels (A-Z with subtypes having an additional number, i.e. Type T4) and a somewhat looser categorization of hilts styles from 1-12 (no subtypes). He also identified thirteen late-period "sword families" based on common combinations of the above elements and most likely dictated by the demands of fashion.
With this standard typology, archaeologists are able to classify a sword quickly and easily, and therefore assign a weapon to a specific time period based on prevalent fashion in blades; For example a Type XIII sword with a Style 6 hilt and a Type J1 pommel would likely be a mid-to-late 13th-century weapon from England, based on blade and hilt for period and the popularity of the J1 style with English swordsmiths.
An extensive review of his classification system can be found at MyArmory.com.