Known as rank cylinders or code cylinders, these objects contribute to the insignia of officers and subofficers. These small silvery cylinders are kept in narrow pockets on either side of the upper chest, near the shoulders. Their practical function is to act as computerised permits to allow the officer access to areas under his authority. The cylinders also can be inserted into the standard computer interfaces which are also used by astromech droids. In this role they provide security clearance for access to sensitive data within the field of jurisdiction of the officer or subofficer.
The number of cylinders worn by an officer supplements the configuration of the rank badge in order to distinguish between officers of close but different ranks. For instance a naval captain and commader can only be distinguished by the number of cylinders; all else being equal, the commander will have one more cylinder than the captain. In general where two officers have the same badge, the one with fewer cylinders is superior.
Due to practical security considerations discussed above, every officer should have at least one cylinder. In some cases unusual distinctions of seniority would require zero or a negative number of cylinders to accompany a badge of a common red and blue style; then gold badge units are introduced to imply what is equivalent to cancellation of a number of the cylinders which are physically present.
Cylinders perform a similar role distinguishing among the black-suited subofficers of both the Imperial Army and Navy. However, since few of these men have been named as subjects of existing documentation, we have only the weakest grounds for commenting on the details of their subset of the insignia system.
The aesthetics of the insignia system seem to require that cylinders be positioned at the shoulders in a certain order. The first cylinder is placed at the left shoulder; the second goes at the right. A third cylinder is placed at the left shoulder. There is never more than a fourth cylinder, due to crowding of the pockets. When present, this last cylinder is placed at the right shoulder, giving a total of two cylinders on each side. Differentiation equivalent to the addition of any further cylinders is accomplished via an appropriate equivalent modification of the rank badge (where present).
There are two physically distinct forms of rank cylinder. The usual kind is about a centimetre in diameter, has a dark bulbous surface at its tip and a small pocket clip to prevent the cylinder from becoming dislodged and lost. The second kind is somewhat thicker and is structured with several concentric shells exposed at the top. Where the common cylinder type resembles a pen, this kind resembles a spark-plug. The "sparkplug" kind has only been seen in one instance, to my knowledge: Admiral Motti aboard the original Death Star.
Common pen-shaped rank code cylinders, as worn by Grand Moff Tarkin.
Uncommon sparkplug-shaped rank code cylinders, as worn by Admiral Motti.
At certain points in the films, some colouration is faintly visible at the tops of the pen-shaped rank cylinders. This might be meaningful. Unfortunately the coloured markings are too indistinct in most scenes for the determination of their actual significance. Most of the cylinders with visible colour seem to be blue.
STAR WARS The Visual Dictionary is the first official publication to show closeups of Imperial officer rank cylinders. Cylinders in the styles of Tarkin and Motti are shown, and also another two varieties for stormtrooper officers, The tops of the latter cylinders resemble Tarkin's kind, but there is a different pocket clip and the shafts are shorter. One of the stormtrooper cylinders has a partially black casing; the other is almost entirely silvery.
Mathew Clayson [Hi-Impact Props & Costumes] provides some information about the real-world, extrinsic nature of the props representing Imperial officer code cylinders:
"I can also confirm that the Empire code cylinder is a red source flashite/torch. The black and silver cylinders in the Visual Dictionary were chalk holders that I provided to David West Reynolds."
Jeff Allen [ Definitive Stormtrooper Costume How To] has attended the Magic of the Myth exhibition in Chicago, Illinois USA, where he made close photographs of the Tarkin-style rank cylinders. Jeff kindly sent the photos below. Features of these images include: the difference between metals of the cylinder and its clip; the blue hue of the cap section. A vertical streak appears runs along the side of the cylinder about ninety degrees from the clip; however it is a figment of curved reflections, and not a physical seam. There does appear to be a transverse seam beneath the clip, suggesting that the cylinder has a removable cap, like a pen's lid.