Depending on their role in a bolted joint, screws may function as fastening mechanical elements or as lead screws.

Screws and their connections – such as a through-hole bolted joint with a hexagon head bolt, a flat washer, and a hexagonal nut – serve the purpose of fastening. During operation, the screw connection prevents the joined components from any relative

• relative displacement between the connected parts,
• rotation,
• and separation.

Screws can have either:

• left-hand thread, or
• right-hand thread.

In most cases, right-hand threads are used. Left-hand threads are required when differentiation is needed (e.g., for safety reasons or to avoid incorrect connections). For example, flammable gas cylinder connectors use left-hand threads to prevent them from being mistakenly connected to oxidising or inert gas systems.

The thread shape is defined by its thread profile. An external thread type can only mate with an internal thread of the same type. Dimensions of external threads (e.g., bolts) are denoted by lowercase letters, while dimensions of internal threads (nuts or tapped holes) are denoted by uppercase letters.

The most commonly used thread type is the metric thread, characterised by a 60° flank angle. It is denoted by the letter 'M', followed by a number indicating the nominal diameter.

Based on the number of starts, screws may have single-start or multi-start threads. In the case of:

a  Single-start  thread, one helical thread begins along the circular cross-section. With a multi-start threaded spindle, several threads start simultaneously along the circumference with the given pitch, following the helical path.

 Multi-start   threads have a greater lead angle to allow enough space for the multiple threads around the diameter. 1 start = 1 thread, 2 starts = 2 threads, 3 starts = 3 threads.