Metal tape measure 

Measure width from the outer edge of the frame on the left to the outer edge of the frame on the right, then add 100mm (i.e; 50mm to either side).​​

For the height (drop) measure from the top of the frame to the lowest point of the frame, then add 150mm (i.e; 100mm above the top of the frame and 50mm below the frame).

Choose your chain control side. Usually the most convenient side to get to

Chain colour

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Beige/Black/Grey/White/Nickle Plated