explain the two tier system of convertibility under brettonwood exchange rate system.

The above topic has already been covered in our study material. Follow the link to view the same and scroll down to the topic "The Bretton Woods System of Exchange Rate".

https://www.meritnation.com/cbse/class12-commerce/studymaterial/economics/introductory-macroeconomics/open-economy-macroeconomics/341_1991_5853#slide1_Meaning and Type of Exchange Rate, Determination of Exchange Rate