The CD (Consolidated Design Numbers) system was begun almost 35 years ago. The need for it arose from the fact that various manufacturers used different numbers to identify the same insulator: and further, sometimes a manufacturer used the same number to identify different styles at different time periods. For example, Hemingray No. 14 identifies three entirely different styles of insulators, depending on when they were made.

Moreover, many of the older insulators were not marked with a style number; and while most of them were probably so identified in sales literature at the time, many of these are not now known. In order to facilitate a system of identification it became obvious that a unified set of numbers should be applied.

In practice, an insulator is not a precise item as is, for example, a machine part. Other than the desire for a reasonably good fit on the pin (even this was often not achieved in earlier production), minor variations throughout make little difference in use. Therefore, we have an endless variety of slightly differing pieces. Because of this, it was never thought practical to devise a precise numbering system. The CD numbers are in fact intended only to classify the insulator in approximate terms. To carry the system beyond this and attempt to give each variation a number would not only result in assigning thousands of numbers; it would also necessitate the taking of accurate measurements of each insulator to be identified. The CD numbers were intended to include only such variations as would be visually obvious, without special skill or tools.

In some instances, notably those involving styles that were popular over a long period of time, there are indeed variations within one CD number that are most obvious visually. CD numbers 112 and 145 are examples. The problem here is that, although extreme specimens are quite distinct from one another, there will be examples spread between the extremes that measure in increments so slight as to he most difficult to distinguish. For this reason, it works better to allow one number to cover all. Where more information is required, accurate measurements or other description must be included.

In using the CD system to identify individual insulators, scale drawings and numerical cross reference lists have been provided. It usually is easier to start with the insulator manufacturer number and check it against the manufacturer number index. For example, if you have a Hemingray No. 16, your cross reference will indicate CD Nos. 121 and 122. It is then a simple matter to check the scale drawings and see whether you have one of the earlier, narrow groove No. 16 (CD #121); or a wide groove 16 made after 1921 (CD #122).

Where there are no manufacturer style numbers on the insulators (as is the case with most Brookfields and other older ones) it is necessary to go directly to the drawings. In order to facilitate a search of this type, an attempt has been made to group the numbers so that all of one general class of insulator is together. A guide follows:

Additional to the above listings, numbers in the 700 series have been assigned to early North American threadless styles. Numbers 1000 and above have been assigned to non-pin types.

In the table, there will be variations in some numbers because certain styles that are basically North American have also been manufactured elsewhere. For example CD #154 has been manufactured in four countries that we know of, outside of the North American continent. A few styles that are very similar to familiar North American ones but were made elsewhere have been given decimal numbers and fitted in the North American sequences. Styles in the 600 - 639 group are quite similar to some of those in the 206 - 249 and 290 - 309 groups. One might need to check both areas if an insulator of unknown maker is being examined. The distinction between North American and European styles is general only and is for convenience in a search; but it will not apply in each and every case. There will be crossing between the groups.

Finally, a word about the decimal numbers. These have been used to increase the capacity of the system without altering originally-assigned numbers. In most cases, the decimal number is similar to the preceding one; for example, CD # 203.2 is an experimental version of CD #203. In some cases however, entirely dissimilar pieces were given decimal numbers because there was really no good place to put them otherwise. Most of these have .8 or .9 designation, for example CD #138.9, the "Twin-pin". Most of these will be obscure styles that never gained general acceptance.

Once again, the CD system is not precise. Find the closest number, then describe variation in design along with color and other details. It's the best we can do without making the system unduly cumbersome.