A rivet is small, bolt-shaped joinery device, used to connect metal elements. Typically, rivets are made of aluminium, copper, iron or steel. The rivet's head is formed through pressure and, for iron and steel rivets, with the application of heat. You can differentiate among types of rivets by their varied head and tail shapes, each form ideal for distinct applications.
It's very likely you've seen round-head rivets, a commonly used head style. As the name indicates, rivets with round heads have a bulbous, arched shape, more pronounced than among brazier head or universal head styles. Sometimes the round head is also referred to as a "cup head." The round head may be unmarked or it may feature a dimple or a raised dot, raised double dashes or a raised cross.
Instead of bulging outwards, a countersunk rivet has a broad, flat, circular face. From the flat face, a countersunk head tapers down dramatically to meet the shaft. If you look at a countersunk rivet from the side, the profile of its head resembles a trapezoid, with its short side equal to the width of the rivet's shaft. The angle between the two sloping sides is roughly 90 degrees. Like the round head rivet, a countersunk head may also feature a number of markings on its outer face, including a dimple and a raised dot, double dash or cross.
The universal rivet head is another very common form. At first glance, it may appear similar to the round head, especially if viewed head-on. However, unlike the round (or cup) head, the universal head's profile is not continuously curved. Viewed from the side, its head appears like a slightly flattened arc; while the head does curve, it flattens out slightly at the centre. The flat face is approximately as broad as the diameter of the shaft. If you're walking on a riveted plank and you step on a universal rivet, you may be able to discern its flat surface, compared with the round head. Like the other rivet head types, you can find universal heads with unmarked or marked faces.
Like round-head rivets, brazier heads are smoothly curved. However, when viewed from the side, brazier heads do not form a complete semicircle. They have a lower profile, with a broader curve. When seen riveted into place, brazier heads do not protrude as much as other rivet head types. By comparison, a 3/32 size countersunk head measures around .04 inches tall and .163 inches wide, whereas a 3/32 size brazier head measuring .156 inches wide would be only 0.031 inches tall.