Roulette

Rouletting is a series of small cuts to allow stamps to be separated from a sheet or pane or from each other. If you've seen a sewing/quilting/dressmaker's pounce wheel or tracing wheel tool, the resulting pattern would be pretty much the same.

In early times where imperf issues were still common, pounce wheels have been used privately or unofficially for separating them. For example, the United States' Kansas City roulette was used at that post office in late 1914 to more easily sell an accumulation of imperf stamps. Private roulettes usually cannot be traced back to the original user/maker unless found on cover with a company corner card or cover contents.

Otherwise, rouletted stamps were normally created by sharp pieces of metal with a dash pattern to cut through a whole sheet at one go on standard printing presses of the time, just like tickets were and are produced. Certain issues had the cutters inked to help with separating stamps. That roulette color is in the stamp color implying that printing and rouletting was done in one step.

That uniform rouletting are measured just like perforations, treating the uncut sections between cuts like perforations.  File:Wurtt 7kr roul.jpg|Wurttemberg 1868 7kr rouletted. Note the rounded corner, showing a typical problem in separating rouletted stamps. It was probably not a problem for the original user, but is for collectors looking for absolutely sound stamps. File:Luxembourg arms roul.jpg|Luxembourg 1872 25c rouletted in color. File:Greece 1L serrate.jpg|Greece 1L value. Separation into single stamps usually doesn't happen this cleanly. File:Finland serp roulette.jpg|Finland 5k value. Note the tiny projections on each tooth, the uncut parts that kept a sheet/pane from falling apart. These caused the common broken teeth when stamps were separated from the sheet, but again, this was not a major concern for stamp users at the time. But it has become one for collectors demanding perfection. 

Compare with the uncommon hyphen hole perf, where rectangular holes are cut into the paper. However, those stamps can be hard to tell apart from rouletting on individual stamps.

Note that because the roulette cutters can dull quickly, it is fairly impossible to insure that all cuts are done solidly. It was not always the perfect way to allow separation of stamps. Impatient or frustrated postal clerks or stamp users of the time would then resort to scissors to separate stamps, just the thing that rouletting was intended to solve. So the characteristic nubs seen on the edges of rouletted stamps will tell you that the stamp has not been scissor-trimmed and thus damaged.