Basic Tobago Gridlayout Question

[Robert D. Morse]

Hello,
I'm making the slow transition from Struts to MyFaces and Tobago.    
Could someone point me to an explanation of the tc:gridLayout  
component?   Specifically, the syntax for the "columns" and "rows"  
values.   For example, I'm not really sure what 'columns="1*;4*"  
does, or when to use "fixed" vs a specific pixel width.   The tag  
docs just seem to give the various values but no real explanation.