Two-piece straps are the de facto strap set-up. As the name would suggest, two-piece straps comprise of two parts, which can be referred to as the "head" and the "tail." The head of the strap is normally shorter and carries the buckle. As well as the buckle, the head normally carries two keepers, the loops that hold the tail of the strap in place once it has been threaded through the buckle. The first keeper is almost always fixed, or at least restricted so it cannot move around too much. It sits close to the buckle and is the first keeper the tail-end passes through. The second keeper "floats", enabling the wearer to tuck the tip of the tail away wherever it may fall on the wrist.
The tail of the strap is punched with several holes. The pin of the buckle passes through these holes when fastening, adjusting the size of the strap on the wrist.
An ardillon buckle, similar to the buckle used on a belt, is common. When a different type of buckle is used, the roles of the head and the tail change slightly. Sometimes, in situations where the tail must to cut to the correct length, for example, there are no holes on the strap whatsoever.
There are many different types of two-piece straps. They can be made from leather, rubber, fabric, and even metal. Often, two-piece metal straps are made from a fine mesh, but examples with links or even chunkier, shark-mesh are possible.
The NATO strap has its roots in the single-piece, 16mm strip of tropicalized webbing used by troops during World War II to secure their timepieces to their wrists. By threading the single-piece strap through the spring bars (and thus behind the caseback) the watch was not only held snugly and securely against the wrist, but also would remain attached should one spring bar fail. The popularity of this strap style meant it eventually became part of the official kit requirements for British service people in 1973.
There are two styles of NATO straps, one for the army/navy and one for the RAF. Each was given "NATO Stocking Numbers", or NSNs, leading to the nickname. For some purists, the strap should be referred to as the G10 after the G1098 quartermaster's store from where service personnel collected their gear.
Now, NATO is often used to describe any style of strap that is fed through the spring bars, with a few variations in terms of hardware profile and ring number. A 3-ring NATO is modeled after the earliest style of strap, popular among soldiers and pilots in WWII. It features an ardillon (pin and tang) buckle and two fixed metal keepers for the tail-end of the single-piece of webbing to be threaded through and then back on itself.
4-ring NATO straps are perhaps more common these days. They have an extra flap of material that sits between the spring bars and the skin, adding an extra layer of security to the set-up. The fourth ring is for the tail-end of the strap to be threaded through before it is passed through the buckle and keepers.
While there are some similarities, a NATO strap is distinguished from a ZULU strap by the shape of its metal hardware. NATO straps have squared loops and a flat, angular buckle, which sits very close to the wrist. Meanwhile, ZULU straps tend to much thicker and more heavy-duty. Rather than using angular, low-lying keepers, the ZULU style prefers large metal loops and a very simple buckle design that matches the keepers exactly.
The 3-ring and 4-ring ZULU straps follow the same threading pattern as their NATO counterparts, but the 5-ring version gives wearers an extra option when it comes to threading their strap. Rather than passing the tail-end of the strap directly through both rings on the end of the underside flap and straight into the buckle, the tail-end can instead be threaded through both loops and then back over the second loop and through the first again before being pulled tight and threaded through the buckle. This provides a very secure fit for the watch head and may be preferable for some wrist profiles.
Because of their thickness, single-piece 3-ring ZULUs are a popular choice for adventure enthusiasts that want the look and security of a NATO but increased durability while retaining a low-lying profile.
An off-shoot of the NATO, the USM (which stands for United States Military) works in the same as the original single-piece NATO straps, but replaces the metal keepers with a thick, free-floating fabric loop.
Tropical straps are quite uncommon but very useful for those spending long periods in incredibly hot and humid climates. These straps are distinguished by their many perforations and use as little material as possible to keep the watch on the wrist. They are normally made from sweat-resistant materials such as rubber. Very few manufacturers make these straps as standard, but they can be picked up from after-market sellers quite freely and used to give classic timepieces a new, more dynamic look.
Racing straps are generally made of leather, quite heavily padded, and perforated with a series of circular holes that decrease in size the further away from the watch head they get. These holes were supposedly added to increase airflow to a driver's wrist and prevent discomfort during racing.
While not extremely common, some straps use the same tightening method as airplane seatbelts and waistcoats. These straps tend to employ a USM-style fabric keeper as the tail-end is often quite long. The advantage of these straps is that they offer a perfect fit, as the length of fabric that can be adjusted to the micron thanks to the buckling system.
Unfortunately, what they offer in fit they often lack in elegance. A strap of this nature is excellent for a tool watch, but in formal settings such a strap would not sit well at all.
As well as rubber/leather/fabric straps, there are also metal bracelets to consider. Bracelets can be made from precious metals and even be set with precious stones. Good, integrated bracelet designs are often as famous as iconic watch references themselves.
Even if your watch does not come with a metal bracelet, you can choose to go with a generic after-market option. They can be added to almost any style of watch to change its character. Although commonly marketed on "Sports Watches", bracelets skew formal, and are, in most cases, better suited to the office than physical activity. The real exception to this rule is when it comes to diving, with metal bracelets often favored for their extreme durability and reliability under duress.