Folding rule

How it works

The rule is built from two or more sections of equal length joined end to end by pivots. Folded, the sections lie against one another and the rule is carried in a pocket. Opened, the sections are brought into line and the graduations along the edge read as one continuous scale.

In the older arch-joint and square-joint rules the sections are held in line by the fit of the metal joint plates and by the user’s hand. In the spring-jointed rule that Stanley sold as the Zig-Zag, Stanley’s 1900 catalog says each joint contains a stiff spring that holds the rule rigid when open. Bodmer and Schade’s 1913 patent for Stanley states that the joint is designed to keep the graduations accurate along the whole length and to give an unbroken line of divisions and an unbroken ruling edge.

The rule is a regulated measuring instrument where it is sold for trade. The U.S. NIST Handbook 44 requires that a folding measure open to a definite stop and be straight when so opened. The European directive adds that the joints of a folding rule must not add error beyond 0.3 mm for Class II or 0.5 mm for Class III.

Anatomy

Section (leaf): A flat length of boxwood or other hardwood; the Stanley lists offer boxwood and ivory, and the Science Museum Group holds a four-fold rule of brass and boxwood.
Joint: The hinge between sections. Stanley’s 1859 price list distinguishes arch, square and round joints; its 1900 catalog describes blued steel spring joints on the Zig-Zag rule.
Joint plates and bits: Brass or German silver plates that carry the pivot; “bound” rules have the plates wrapped around the edges and “bitted” rules have small plates at the ends of each leaf, in the terms of the Stanley catalogs.
Graduations: Divisions of the inch, commonly eighths and sixteenths in the Stanley lists; some rules add tenths, twelfths, drafting, board-measure or octagonal scales.
Slide or caliper: Some two-fold rules carry a slide, which Stanley’s 1859 and 1900 lists both offer, and some carry a caliper arm for measuring thickness, which the 1900 list offers.

Variants

Two-fold and four-fold wooden rule: The traditional form: a two-foot rule folding once, or a one-foot or two-foot rule folding in four, in boxwood or ivory.
Zig-zag (snake) rule: A rule of many leaves, each joined to the next by a spring joint so that the whole folds back and forth in a stack. Stanley registered the name Zig Zag for it in 1899, according to the Tool Shed.
Caliper rule: Four-fold and two-fold boxwood and ivory rules with a caliper graduated in sixteenths or thirty-seconds, listed in the 1900 Stanley catalog.
Blindman’s rule: A Zig-Zag rule with extremely large figures, added to Stanley’s regular line by 1900.
Architects’ and board-measure rules: Rules with bevels carrying drafting scales, or with tables on the inside of the leaves that give the board measure of one-inch boards.
Blacksmiths’ rule: Two spring-brass legs joined by a brass joint, which Stanley says can be cooled by plunging in water without rusting.
Metric and mixed-unit folding measures: The Science Museum Group holds one-meter folding measures marked with regional inch units and with centimeters.

Specifications

Open length · ft or m: 2 ftTwo feet is the length of the joiner’s rule that Moxon describes in 1693; the Stanley lists also give one-foot, six-inch and three-foot rules, and the Science Museum Group holds one-meter folding measures.
Number of folds · count: Two-fold and four-fold in the Stanley lists; the spring-jointed Zig-Zag has more leaves. No standard distribution has been sourced.
Joint error (EU) · mm: 0.3 mm (Class II), 0.5 mm (Class III)Maximum additional error allowed for the jointing of a folding rule under Annex X of Directive 2014/32/EU.
Maximum permissible error (EU) · mm: Set by accuracy class by a formula in the directive; the formula and class constants are given in the tape measure entry.

Made to

NIST HB44-5-52
EU MID-MI-008
OIML R.35

Safety and care

Care

Stanley’s 1900 catalog describes the blacksmiths’ rule as made of brass so that it can be cooled in water without rusting; no care instructions for wooden rules have been sourced.

Tips

Numbers run up on the inside of the Zig-Zag
Stanley’s 1900 catalog says the numbers on its Zig-Zag rules are placed to “run up” on the inside of the rule. The company states that this lets the user take a short measurement without opening the whole rule while keeping the graduations close to the work.1
Board-measure tables on the leaves
The catalog lists a two-foot, four-fold boxwood board-measure rule whose inside tables give the contents in board measure of one-inch boards of ordinary dimensions. The user reads board feet directly instead of calculating them.1
The caliper arm measures thickness
Stanley’s caliper rules carry a caliper graduated in sixteenths or thirty-seconds of an inch, in boxwood and ivory and in one-foot and six-inch lengths. It gives the thickness of stock or the width of a rebate from the same tool that measures length.1
Read an extension rule from the left-hand large figure
For the extension rules in the same catalog, the user adds the feet shown by the large figure nearest the left end to the inches and fractions exposed from under the left end of the upper section, and may lock the sections with the set screw to carry a measurement between two fixed points.1
Confirm the rule opens to a stop and lies straight
NIST Handbook 44 requires that a folding measure open to a definite stop and be straight when opened, and the European directive limits the additional error from the joints. A rule that bends at a joint or can be opened past straight does not meet either requirement.1 2
Use a straight rule to test flat work against light
Moxon (1693) describes joiners testing work by laying an edge of the two-foot rule on the wrought face and bringing the eye close to see whether light shows between the edge and the work. If no light shows they conclude the work is true.1

History

Earliest documented

Roman period (the records do not give a closer date)

Roman Britain (fortress pit at Usk, Monmouthshire; a rule in the London Museum)

Amgueddfa Cymru holds fragments of a bronze folding foot-rule excavated at Usk in 1973 from a fortress pit. One arm is almost complete with evenly spaced notched dots on two surfaces, and a stay pivoted from one arm to lock on rivets in the other to hold the rule rigid in use. The museum notes that the Roman foot equals 0.295 m. The London Museum records a Roman copper-alloy folding foot rule with a rivet, 295 mm long open and 150 mm closed.1 2

The folding rule solves one problem: a straight scale long enough to be useful is too long to carry. The hinged foot rules from Roman Britain solved it with two arms, a hinge and a pivoting stay that locked on rivets. The London Museum rule measures 295 mm open, which matches the 0.295 m that Museum Wales gives for the Roman foot, and 150 mm closed, so it folds to about half its length. The records cited here therefore place the hinged rule at least as early as the Roman period.

The jointed wooden rule of the early modern carpenter took its materials from the shop. Moxon’s 1693 account of the joiner’s rule describes a rule two feet long, divided to eighths of an inch and carrying board and timber measure, which indicates what the trade expected a rule to carry. The Science Museum Group holds a brass folding rule of about 1700 and a folding foot-rule with scales of 1780–1790, which show that hinged carpenters’ and scale rules were made in England in the intervening centuries.

By 1859 the Stanley Rule and Level Co. offered boxwood and ivory rules in an organized range: two-fold and four-fold, arch, square and round joints, bound and plain, with slides, scales and calipers. Its 1900 catalog still lists specialized wooden styles, including caliper, architects’, board-measure, blindman’s and Gunter’s slide rules, alongside blacksmiths’, extension and pattern makers’ shrinkage rules. The range shows a folding rule made in many forms for particular trades.

The spring-jointed zig-zag rule is the one significant change in the form, and its origin is stated differently in different sources. The only primary record located for the joint is German patent 41417, filed in 1886 by Gebrüder Ullrich for a jointed-rod device with spring locks. Stanley’s 1900 catalog says the general form is primarily of German design and claims its own features: a joint that needs no hole through the stock at the joint and numbers that run up on the inside. The Tool Shed says Stanley bought the American rights in 1898 to a design by Oscar Schubert of Berlin, called it the Zig Zag, registered the name in 1899, and made at least 23 patented improvements between 1901 and 1935. Bodmer and Schade’s 1913 patent, written for Stanley, calls the type the “snake” or “zigzag” rule.

The rule also became a regulated instrument. NIST Handbook 44 requires that a folding measure open to a definite stop and be straight, and the European directive names the folding type specifically and limits the added error from the joints. According to the Time Tested Tools compilation, Stanley introduced its first steel tape rule in 1931; the folding rule and the tape measure have been sold side by side since, and the sources reviewed do not measure how one displaced the other.

Timeline

Roman period
Hinged copper-alloy foot rules are in use: a stay locks the open rule on rivets (Usk) and a London Museum example measures 295 mm open and 150 mm closed.1 2
1693
Joseph Moxon describes the joiner’s rule as commonly two feet long, with each inch divided into eight parts and with board and timber measure marked on it.1
about 1700
A brass one-foot folding carpenter’s rule of about this date is in the Science Museum Group collection (object 1938-326).1
1780–1790
A folding foot-rule with scales of this date is in the Science Museum Group collection (object 1959-178).1
1859
The Stanley Rule and Level Co. issues a price list that offers two-fold boxwood rules with arch, square and round joints, bound and slide variants, and ivory rules, after the union of A. Stanley & Co. and Hall & Knapp.1
1886
Gebrüder Ullrich of Maikammer, Rheinpfalz, file German patent 41417 on 10 September 1886 for a jointed-rod device with spring locks; it is published on 2 December 1887.1
1898
According to the Tool Shed, Stanley buys American rights to a spring-joint rule designed by Oscar Schubert of Berlin, brought to its attention by Justus Traut.1
1899
The Tool Shed reports that Stanley registered the name Zig Zag as a trademark for the new line of rules in 1899.1
1900
Stanley Catalogue No. 26 lists Zig-Zag rules in several lengths, made of flexible hardwood with blued steel joints and tips, and says the general form is primarily of German design.1
1913
Christian Bodmer and Edmund A. Schade of New Britain receive US Patent 1,080,192, assigned to Stanley, for a folding-rule joint for “snake” or “zigzag” rules that keeps the graduations accurate and unbroken.1
2014
Directive 2014/32/EU sets accuracy classes for material measures of length and limits joint error on folding rules to 0.3 mm (Class II) and 0.5 mm (Class III).1

Where the sources disagree

Who designed the spring-jointed folding rule that Stanley sold as the Zig Zag?

Gebrüder Ullrich of Maikammer filed German patent 41417 on 10 September 1886 for a jointed-rod device with spring locks. The record names the firm, not an individual, and the text of the claims has not been read here.1
German patent record DE 41417 C
Oscar Schubert of Berlin designed the spring-joint rule; Stanley acquired American rights in 1898 after Justus Traut brought it to the company’s attention.1
The Tool Shed (EAIA chapter newsletter, 1997)
The general form is “primarily of German design”; Stanley claims only particular features as its own and names no designer.1
Stanley Rule and Level Company, Catalogue No. 26 (1900)

When did Stanley introduce the Zig-Zag rule?

Rights acquired in 1898, trademark registered in 1899.1
The Tool Shed
Stanley introduces the Zig Zag rule in 1903.1
Time Tested Tools & Forge timeline (collector compilation)
Zig-Zag rules are already offered and illustrated in the August 1900 catalog, which also says a complete department of the factory is devoted to them.1
Stanley Catalogue No. 26 (1900)

Gaps in the record (6)

The claims of German patent 41417 have not been read, so the content of the Ullrich patent beyond its title and dates is not established. The record names Gebrüder Ullrich; Anton Ullrich is not named in the record read here.
No source located connects Oscar Schubert to a patent number; the Tool Shed account is a collectors’ newsletter.
The earliest hinged rules are known only from museum catalog entries; no dated archaeological report was located, so the Usk and London examples are dated only as Roman.
Care of wooden and brass folding rules has no sourced entries.
No standards-body figure for common accuracy of wooden folding rules was located beyond the EU class formulas.
No sourced figure for how folding rules compared with tape measures in use.