  We present the new absolute frequency measurement of ytterbium (171Yb)
obtained at INRiM with the optical lattice clock IT-Yb1 against the cryogenic
caesium (133Cs) fountain IT-CsF2, evaluated through a measurement campaign that
lasted 14 months. Measurements are performed by either using a hydrogen maser
as a transfer oscillator or by synthesizing a low-noise microwave for Cs
interrogation using an optical frequency comb. The frequency of the 171Yb
unperturbed clock transition 1S0 - 3P0 results to be 518 295 836 590 863.44(14)
Hz, with a total fractional uncertainty of 2.7e-16 that is limited by the
uncertainty of IT-CsF2. Our measurement is in agreement with the Yb frequency
recommended by the Consultative Committee for Time and Frequency (CCTF). This
result confirms the reliability of Yb as a secondary representation of the
second and is relevant to the process of redefining the second in the
International System of Units (SI) on an optical transition
