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We carry out investigations of inverted crossover resonances (ICRs) in π-driven four-level systems where can be zero. Through the use of sub-Doppler frequency modulation spectroscopy of the − transition in 171Yb the resonance becomes manifest. The centre frequency is inherently insensitive to first-order Zeeman shifts and equates to the two-level resonance frequency in the absence of a magnetic field. A rate equation model is used to help validate the nature of the resonance. Optical frequency measurements of the hyperfine line recorded over two months demonstrate a statistical uncertainty of 2 × 10−11. The ICR found with the line is used for 556 nm laser frequency stabilisation, which is an alternative means when applied to magneto-optical trapping of 171Yb. [ABSTRACT FROM AUTHOR]