Seax

A seax was a type of knife that was a very common, single edged blade, popular with the Norse, Franks, and Germans during the early middle ages. Pronounced "Sax", it is the basis for the term "Saxon" in "Anglo-Saxon". They have small cross pieces or no crosspieces on the hilt. The edge either curves toward the tip which is in line with the middle of the blade (in earlier, smaller seaxes) or the edge is straight and the back descends at a steep angle toward the tip which is at the end of the cutting edge (in later, longer seaxes).
There are a number of different designations such as the "narrow" seax, the "broad" seax and the "long" seax, that get their name from the shape of the blade. Brendan's seax is a long seax with a blade of 20 inches. Total length is just over 24 inches. Their design made them excellent stabbing weapons and good slashing weapons. The long seax was big enough that it was often carried as the primary edged weapon of Germanic and Norse warriors in the early middle ages.

The knives in the picture are narrow seaxes but show a common feature in the back angling down towards the blade.