dot in variable name

[Eric Peterson]

I remember reading about variable names containing dots (i.e.  tax.amount).  
Some sort of namespace of variables. 

$ declare tax.amount=0
-bash: declare: `tax.amount=0': not a valid identifier


I can't find any link to it anymore.  I could be wrong. But is this allowed in 
some manner?  I like the idea of grouping logically related variables by 
namespace.  Not sure how or where I'll use it, but seems like an interesting 
methodology.

Thanks
Eric