In Rs 10 and Rs 20 coins, the metals of central pieces and the annular rings have been swapped.
Rs 10 has outer ring of Nickel Brass (yellow) and central portion of Nickel silver.
Rs 20 has outer ring of Nickel Silver and central portion of Nickel Brass.
GoI must have done this to avoid wastage of metal. Central portion of Nickel Brass which was cut out from Rs10 coin can be used as central piece in Rs 20.
My friend Pabitra ji may throw some more light on world wide practices.
Regards
Sanjay Kansal
Sanjay ji,
Indian mint seems to have borrowed the concept from Mexican mint but only partially
See reply #2
http://www.worldofcoins.eu/forum/index.php/topic,36414.0.htmlTill the core size is indicated or measured, one can not be sure.
Regarding swapping, it is normal for countries to have one coin with yellow ring and other one with white, whenever the country has two Bimetallics.
Very few countries have same colour bimetallic ( Algeria 10 Dinars KM#124 is only one) or more than two bimetallic coins ( Kenya and Mexico are the only one left, Mauritania shifted to one trimetallic).
Theoretically, the preferred choice was White ring which used to be Copper Nickel since that alloy has high wear and tear resistance but now that principle appears to have been given a bye in view of plated steel rings and cores.