Source Genre: Legal Collections

Legal validity: The Sachsenspiegel laid out the rules for the imperial elections (technically for the election of the 'King of the Romans'). The King of Bohemia, was excluded from the election because he is "not a German". But in fact the King of Bohemia participated, as proven by reliable documents, in most elections after 1122. On the other hand, this clause was brought to bear on the electoral process during the Braunschweig 'second election' of 1252, when the north German princes and barons refused to accept the election of William of Holland because the two north German princes, the Duke of Saxony and the Margrave of Brandenburg, both qualified according to the Sachsenspiegel to take part in the election, had not done so. Unlike the King of Bohemia, these north German princes lived "in an area the laws of which had been codified by the Sachsenspiegel" (Kaufhold). This example shows that a legal text like the Sachsenspiegel were easy to cite to legitimate one's position when one wanted to undermine someone else's claim, but one has to be careful about the extent to which it actually was valid in regular practice.

Multiple redactions: We can see how the text of the Sachsenspiegel was adapted to the needs of the day over the years: in a 13th-century manuscript of the Schwabenspiegel (Mirror of the Swabians), the Sachsenspiegel paragraphs about the election were included and adapted to the political situation. In 1273, the King of Bohemia was excluded after the fact as an Elector in the election of Rudolf of Habsburg because he was the only Prince-Elector who did not vote for Rudolf. In order to achieve unanimity, in 1273 the Duke of Bavaria was polled and his vote substituted for that of the Bohemian King. The exclusion of the Bohemian was discussed at the 1276 diet at Augsburg. The Der Schwabenspiegel, which was written at the same time in Augsburg, documents this conflict. In 1291, the rights of the Bohemian King as an Elector were confirmed in a charter issued by Rudolf of Habsburg.

References for further study:
Heidelberger Sachsenspiegel (digitalized version)
Sachsenspiegel-Online (transcription and translation of the manuscript excerpted here)
Dresdner Sachsenspiegel (with detailed notes)