Most standard negatives contain a grey scale and colour patches and a skin tone in the form of a girl's portrait.
These were and are called "lady wedges", or "lady negs", or were even called by the name of the girl used as the model for the flesh tone element. Kodak Limited, Kodak Pathe, and Eastman Kodak all produced these standards too and as the negative was an original it had to be remade every year or so using another girl model.
This means that there are perhaps hundreds of different standard negatives cut into the front [or back] of production negatives. Laboratories that did not make their own negative used the commercially available ones and in Europe from1965 to the late 1980's the "BKSTS Girl" was the commonest [BKSTS is the British Kinematographic Sound and Television Society]. In the USA Eastman's "China Girl" predominated. Television companies also made standard negatives as a method of tying together the printer control with the telecine control, and in the UK Thames Television and Rediffusion had characteristic standard negatives.
Today there are many less interesting standard negatives with just the grey and colour patches and no picture! Some have sound density check positions.
For each and every printer in the laboratory the density reading from a printed standard negative can be plotted on a running or clothesline plot.
In the case of colour film separate red, green and blue readings are taken and plotted.
If the printer has a separate sound gate then some laboratories prepare a clothesline plot for the reading in the sound track area.
Many laboratories dispense with the running plot for printer control and simply keep a logbook for each printer. The running plot with show quickly if a lamp is beginning to fluctuate or diminish in brightness, which is usually a sign that it is about to expire altogether.