Folding rules, sometimes called zig-zag rules or jointed rules, are a series of two or more smaller rule strips joined with hinges. These rules can be folded together to be carried easily, or used in confined spaces. They can also be folded out and used to measure longer distances or take measurements that are further away and difficult to reach.
These rules are often divided into 150 mm (6″), 200 mm (8″) or 250 mm (10″) sections. They are typically marked with either metric or imperial measurements or both. Some folding rules also have other markings, depending on their intended use.
Folding rules can be made from different materials, such as steel, aluminium, fibreglass, plastic or wood. The rule strips are joined with hinges and each strip can be folded against each other to keep it short. Once the strips are opened out they can usually be fixed into place, which creates a long rigid rule.
Folding rules can be used for measuring both small spaces and longer distances. They can be transported easily once they are folded up, making them useful when working in the field.
Folding rules can also be used to measure a point which is difficult to reach. They can be extended out to form long, rigid pieces which can reach across or up without bending very much. Some folding rules can be extended to up to 3000 mm (120″) long, depending on individual rules.
Folding rules can also be used to help determine angles on larger structures or for places which are difficult to measure with a just protractor. Sections of the rule can be lined up with an angle, the angles of the rule can then be measured with a protractor.
Folding rules with more than two rule strips can be folded in such a way that it can be used to quickly draw parallel lines down a surface with a straight edge. These rules can be folded into a the shape of a ‘4’ held upside down with two rule strips at right angles to each other. One rule will line up with the edge of the surface and it can be moved up and down to draw parallel lines.
Some folding rules come with specialist markings, for different uses. For example, masonry folding rules have markings on one side to show the proper dimensions of the bricks and mortar joints needed to lay a structure of a specific height or length. Surveyor’s folding rules are distinctly marked out every 10th of a foot or metre.
Folding rule or tape measure?
Folding rules are considered to be predecessors of modern tape measures, which are long flexible strips with measurements marked on them which are coiled inside a case. They can be extended out or retracted into a coil inside the case.
The tape measure has replaced the folding rule for many uses. However, the latter still maintains its position for some purposes, such as construction, where rigid long measuring instruments may be preferred over flexible ones.
Advantages and disadvantages of folding rules
Folding rules are rigid once locked into place, forming a straight measuring instrument
They can be placed so the gradation marks are flat against a surface, allowing for accurate measurements
When folded down to a smaller size, they can be used in just one hand
They can be used, along with a protractor, to measure angles
Folding rules are not suitable for measuring curves or circles
Over time the hinges can come loose, which will make them lose accuracy as the measurements will change
Wooden rules can change shape if they are exposed to moisture for a long time, making them inaccurate. Some plastic rules can be easily damaged along their straight edge. Steel rules may rust
Advantages and disadvantages of tape measures
Tape measures are available in longer sizes than folding rules
The flexible tape can be used to measure curves and circles, as well as straight lines
The plastics, steels or fabric tapes are made from are resistant to rust
The tape can usually be locked into place at any length, up to its maximum
Most tape measures have a hooked end to grip the edge of a surface
The flexible tapes may not be as accurate for measuring straight lines as a folding rule. There is more chance of them moving when being read
You usually need both hands to take measurements with a tape measure, and sometimes you’ll need a second person. Modern spring tapes will spring back into the casing if not held or locked into place
The shape of the tape in spring measures makes it difficult to lie the tape flat against a surface, making readings less accurate
If the hook at the end of a tape measure is damaged it will affect its accuracy