The U.S. Army contracted two varieties of fighting knives throughout the First World War:
• the 1917 model trench knife with the nine inch triangular blade, and
• the 1918 Mark I trench knife with the 6.75 double-edged flat blade
The 1917 knife was the one that was carried during the war. The conflict had ended by the time it was decided to begin production on the second knife, which saw some use during the Second World War.
This article is illustrated with pictures of both and goes into some detail at to the manufacturers and the various issues that the Quartermaster Corps had to consider in making their decision as to what should be involved in designing such fighting knives.
More articles about W.W. I trench warfare can be read here.