Settlement names in ‑um
Danish place names ending in ‑um are most common in Northern Jutland. They can have one of two origins. Either, they are formed with an ending corresponding to Old Danish ‑hēm, in modern Danish hjem, meaning ‘home,' or, in the case of place names, 'settlement.' Such names include, Uldum and Gislum.
The ending ‑um can also be an original dative plural, preceded previously by a preposition that has caused the inflected form. This is the case with, e.g. Tornum and Husum. The dative plural names have a place marking meaning 'in or by X' where X is the thing the inflected word in the place name is referring to, e.g. Husum means '(in, by) the houses'.
The hēm-names are dated to the period of great migrations (300-600), whereas the dative plural names normally are dated to The Viking Age and the early Middle Ages. It is not always possible to see whether present day's um-names are of one or the other type,but for some of the names, it is possible to make a probable identification. It is, e.g. not very likely that place names containing animal names appear in a dative case, and for that reason a name like Tranum, containing the bird name trane, or ‘crane.' is probably a name formed with the ending ‑hēm.
A few names have a positive interpretation. In a source from 1085, it is visible that, e.g. Smørum is a hēm-name because in this source the name is written Smørhem.
In Scania, the old layers of the language are preserved longer than in the more western parts of the medieval Denmark. Therefore, we can be certain that, e.g. the name Åsum is a dative plural because it is written Asum in the oldest Scanian sources. Names formed with the ending ‑hēm can also appear with endings different from ‑um. Amongst those are, e.g. Lem and Græm.
Pitfalls
This overview of the most important Danish settlement names, provides a tool to place names in time. For instance it is now possible to ascertain that Hvessinge must be hundreds of years older than Glostrup although Hvessinge actually is a part of Glostup today.
However, many place names have a form in present time that does not immediately reveal from which words the name was originally formed. Some of these corrupt names look like other name types, and some have passed through independent changes. By examining the oldest written sources, it is often possible to see from which endings those names were formed originally.
Examples of this are Grænge on Lolland, which is an inge-name, and Klinting in Vestern Jutland, which is an um-name. Further to that Maglemer on Lolland is a tved-name, Gislev on Funen is originally formed from one word, and Græsted in Northern Zealand originally ended in ‑holt. In other words, a place name is never sure to consist of the words immediately obvious from the present day spelling.