  Bicoherence is a way to measure the phase coupling of triplets of Fourier
frequencies. We use this method to analyze quasi-periodic oscillations (QPOs)
in the black hole X-ray binary MAXI J1535$-$571 during its 2017
September-October outburst. The bicoherence provides an interesting new
diagnostic to uncover QPO behaviour and the relationships between QPO harmonics
and broadband noise. The bicoherence pattern of type-C QPOs starts as a 'web'
pattern and changes to a 'hypotenuse' pattern after the presence of type-B
QPOs, indicating that MAXI J1535$-$571 is a low-inclination source. The
intensity of bicoherence also exhibits variations across different energy
bands. We try to explain the bicoherence results in the scenario of a
dual-corona geometry.
