Study of Pairs

Many manufacturers offered "pair" rules. Normally a pair consists of 10 inch and 5-inch rules having the same scale set. Supposedly, 10 inch rule was used at desk, whereas 5-inch rule was for "mobile use". Having the same scale set, one can carry out calculations in exactly the same way.
I suppose there were many engineers who had several rules, But the ones who bought "pair" rules must have had clear conscience and pride to be engineers.

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Faber Castell 2/83N and 62/83N

2/83N

62/83N

The King of the slide rule, Faber Castell 2/83N (Novo Biplex), naturally has it's 5-inch pair, 62/83N (Novo Duplex). They share the same scale set of 30 scales. Most of the scales are "over range scales" which enabled users to read values, without resetting the slide, when the answer is slightly outside of the normal range.

Hemmi 259D and 149A

259D

149A

Hemmi appears not to have created "pair" rules. I will include "almost the same" pair rules here. They are No.259D and No.149A. The front side scales are an exact match. The only differences are that trig scales of 149A are reverse scales (SI, TI) and that 149A lacks ST and DI scales.

Keuffel & Esser 68-1100 and 68-1130(Deci-Lon)

68-1100

68-1130

For a period of time, Keuffel & Esser used the system of type numbers with four digits of numbers followed by "-n", where n denoted the length of the rule. This sytem, at least theoretically, ebables to produce the pair for any rule. Not all of K&E's rules have pairs, for obvious economical reasons. But we can find many pairs in their products.
The first in line is the pair of Deci-Lons; 68-1100 and 68-1130, which share the same exact scale set of 26 scales. Both are all plastic body.

Keuffel & Esser 4181-3 and 4181-1

4181-3

4181-1

Here is a pair of K+E's popular 4181 Decitrig models. Being late models, both are all plastic construction. Each has 21 scales.

Keuffel & Esser N4092-5 and 4092-3

4092-5

4092-3

Here is a 10-inch and 20-inch pair. I would assume that the more accurate 20-inch rules were used by professionals in their offices, and therefore, these two may not have been recognized as "pairs".

Pickett N3 and N3p

N3-ES

N3p-ES

This is Pickett's pair of high end rules, N3-ES and N3p-ES, Power Log Exponential Log Log Duplex which share the same scale set of 32. Furthermore, 5-inch version shares the same cursor and side braces with it's big brother, an example of standardization.

Pickett N803 and N600

N803-ES

N600-ES

Pickett marketted their "Dual Rules". It consisted of 10-inch and 5-inch pair, but not exactly with the same scale set. One example is this N803-ES and N600-ES combination. N600-ES, being much slimmer than N3p-ES, must have been practically better suited for mobile application.

Post 1460 and 1461

1460

1461

Post had "little brother" version of their popular rule "Versa log". The two share the same scale set and the same structure, celluloid on bamboo.

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