Low Square Nuts

Square Nuts are four sided internally threaded fasteners. They are used with externally threaded fasteners to create a clamping load on an object.
Square Nuts are the original style of threaded nuts that were available but metric square nuts have been largely superseded by hexagon nuts.

Low Square Nuts vs. Hexagon Nuts 

Square Nuts have a larger surface area which leads to more contact on the bolted objects. Because of the increased surface contact square nuts are less likely to loosen compared to a hexagon but metric square nuts are also harder to tighten.
Square Nuts require 90° of rotation for new wrenching access. Hexagon Nuts only require 60° of rotation.
Square Nuts have more tool engagement than hexagon nuts and are less likely to have the corners rounded off when being tightened or loosed. Metric square nuts are also easier to grip with pliers or be held in place by hand.

Metric Square Nuts are still used today in assemblies either as a blind nut or in a channel. Low Square Nuts are for applications where there is not enough clearance for a regular square nut.

Metric & Multistandard Components Corp. supplies metric Low Square Nuts in zinc plated steel and stainless steel.