Check out the following explanation:

http://community.middlebury.edu/~harris/piano.html
The section: 'Methods of Traditional Tuning'

'First is the traditional "laying the bearings" or setting a central octave in proper pitch using the time-honored methods which involve attending to the "beats" or pulsations of amplitude (loudness) which accompany two sounds which are being tuned to the piano's Equal Temperament system.'

It explains the procedure of setting the Equal temperament of the piano before proceeding to tuning the bass and treble octaves.

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Note added at 2002-05-17 15:54:13 (GMT)
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Or simply \'setting the temperament. I think the French term emphasises the theory behind Equal Termperament, where all 12 notes of the middle octave are tuned to be of equal distance one from another, before tuning the other octaves.

By the way, Vivan, you made the right choice yesterday for the question \'quarte forte\' (Victoria\'s suggestion). My suggestion of subdominant was not on the money for describing an interval. However, I did see the term \'perfect fourth\' today on a site and don\'t know what the difference would be. There may well be several terms widely used to mean the same thing.