[ic] Separate inventory for different sizes possible?

[sda sda]

How can I have separate inventory quantities for 
different sizes of a product?

For example,   4 for small size and 10 for size large
of T-shirt
When someone orders size small, I want the 
inventory for small size to be 3.

Could someone tell me how I can do this?
Thanks in advance,
Sung


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