From Merriam-Webster online –
Guess: to form an opinion from little or no evidence
Conjecture: an opinion or conclusion formed on the basis of incomplete information
Every so often I hear comments from SCA participants along the lines of ‘all of the Viking Age clothes you see are guesses anyways, so it is fine to do X, Y or Z.’ I don’t have a problem with anyone in the SCA choosing to wear garb that is only loosely based on historical evidence – after all the only requirement to participate in the SCA is that you make an attempt. However, I do have an issue with the idea that a well researched reconstruction is still just a GUESS, so anything goes.
If you compare the definitions above, a guess is based on ‘little or no evidence’ while a conjecture is based on ‘incomplete information’. So to recap, a conjecture is based on some evidence (information) even though the evidence is not complete, but a guess is pulled out of thin air.
It is true that there are very few full garments remaining from the Viking Age, but that doesn’t mean that there is no evidence. It is almost always necessary to fill in the gaps between the bits that we can document from archeological sources. However, if your goal is accurate recreation it is important to base your choices on as much evidence as you can find. If you use period images, saga references, comparisons to earlier or later clothing styles, and comparisons with contemporary cultures to flesh out your recreations, it is possible to develop strong theories for Viking Age clothing styles.
So keep in this in mind – some reconstructions are based on historical evidence (conjecture) and some are not (guesses). Make sure you know which kind you are making.