Tapering is the technique commonly called "Scissor over Comb". The technique is used for cutting hair close to the scalp where a clean, even look is desired. Back before the days of section cutting, tapering was the predominate way to cut hair even in the longer lengths. These days, tapering is considered a barbering technique and is taught as such in the educational institutions around the country.
The hair is lifted from the scalp by the comb, the hair that is protruding through the comb is then cut to the desired shape following the shape of the head and the requirements of the finished style. First the bulk of the hair is removed to an approximate length, for this scissors and comb can be used or clipper over comb. Secondly the hair is worked with the scissors and the comb until a uniform appearance (sometimes not so uniform) is achieved . For the purpose of simplicity, in this article I will write assuming that the cut is a "short back and sides" style. I can cover techniques required for other short styles in another article. I'm sure that all of you know the techniques, but as a scissor manufacturer and designer I will give you the benefit of my experience regarding the correct choice of tools and the easiest way to do the job well.
Lets first discuss the bulk removal or "roughing out" job. Most barbers would also do this with a pair of long scissors (7 inch) and comb. They use big scissors because this job is both very hard on the edges of your scissors and hard on your hands. The larger size puts extra leverage into the cut, making the job easier and faster, if the scissors have a serrated edge this also helps the scissor grip and cut larger amounts of hair without slipping.
To the barbers reading this article I would suggest that it is easier to do this job by using clipper over comb (towel dried, and preferable to most barbers) or vertical section cutting while the hair is still damp. Changing to either of these techniques will save you a vast amount of money on scissor servicing as bulk hair removal with scissors over comb is the hardest on scissors of all the barbering techniques. If you are suffering R.S.I type injuries such as Carpal Tunnel Syndrome, then definitely change to a clipper.
O.K. now that the rough shape is there, it is time to taper the hair to a smooth, even contour or taper, hence the name tapering.
Cut close to the comb but do not touch the comb unless the style is very short, you risk cutting into the comb and damaging your scissors. Cut about 0.5 cm away, if the hair is slipping, then you need to have your scissors adjusted or serviced.
The Right Size Scissors
I don't see any reason to use 7" scissors for this job unless you have very large hands, in fact such a large size can be dangerous and awkward in the hands of an inexperienced hairdresser. The ideal sizes are 6" or 6.5" at least half a size longer than your normal cutting scissors.
You may find that a pair of serrated edge scissors is preferable for close tapering which means the scissors are limited for other techniques. Again it seems that not all scissors are designed for the same tasks, your friendly scissor expert will ask you about the techniques you use so he/she can help you to achieve the right set of scissors for you.
Not all hairdressers should have the same set of scissors, in choosing an ideal set you should consider not only the techniques you use, but how often you use them, the size of your hands and many other variables (get good advice!).
I have to say that hairdressers who use 4.5" scissor for tapering are not doing themselves any favours, the task will take longer and may not be as precise. Buy yourself a pair of 5.5" scissors as tapering scissors which you will also find come in handy for other cutting techniques, for example, long line work such as even length styles.
One last point, it is not cheating to finish your tapering job with a good pair of thinning scissors, this will slightly break up the shape and create a softer look as if the hair has grown back for one week.