3.6 mm. Any violin (viola-, cello-) case should contain a tuning fork!

tuning fork c2-523.3 Hz., 3,6 mm., round, nickel plated

tuning fork g1-392.0 Hz. 4x8mm, nickel plated

Tuning fork a-440 Hz. on wooden resonance case, 4x8 mm., with striker

tuning fork e1-329.63 Hz. 4x8mm nickel plated

Often used in older music. 4.5mm

tuning fork e1-329.63 Hz. 4.5mm round, nickel plated

Tuning fork, 4x8 mm., square, nickel plated, d-sharp1-311.13 Hz.

432Hz,  4.5mm

Tuning fork, nickel plated, c#1-277.18 Hz. 4x8mm

Tuning fork c2-523.25 Hz 4x8mm square

Tuning fork, 4x8 mm., square, nickel plated, a#1-466.16 Hz.