Which nail shape is characterized by a straight edge across the free edge?

The square nail shape is defined by its straight edge across the free edge, providing a clean and angular appearance. This shape is often favored for its modern, edgy look and is popular in a variety of styles, from classic to contemporary. The straight edge not only gives the nails a more robust look but also allows for greater durability, making square nails less prone to breakage compared to more rounded shapes. The square shape can also make fingers appear longer and is suitable for a wide range of nail art designs.

In contrast, the oval, round, and almond shapes all feature rounded edges rather than a straight line across the free edge, which is what differentiates them from the square shape. Each of these other shapes has its own unique characteristics and aesthetic appeal, but none exhibit the straight edge feature that identifies square nails.