When it comes to visualizing items with diagonals and straight lines, often the first thing that comes to mind are blades. Blades have been used since the dawn of man, and have been made out of a variety of materials, such as stone, obsidian, steel, and (surprisingly) wood! Blades serve a variety of purposes, with warfare being only one of the multitudes of possible uses. Blades are usually of a roughly straight, elongated shape, with curves and sharp, straight edges being incorporated during the manufacture of the blade. Of the blades featured in this post, the majority are of a mostly straight edge. However, the kris blade incorporates many curves into its construction, which largely serve to cause bleeding damage due to serration in flesh during combat. The shape of the blade itself will result in different effects and will lead them to be utilized in several different ways. For instance, a straight, double-edged, diagonal blade leading up to a point would likely be used for piercing, a blade with a single sharp edge would likely be used for cutting, a serrated blade for causing extra internal damage, and so on and so forth.
https://en.wikipedia.org/wiki/Blade
https://en.wikipedia.org/wiki/Serrated_blade
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