how does null_maps map to columns 

i have read your document:

The first bit is always set and column bits start a 1 offset in the null map. If a bit is set for the current column then this indicates the the default is null (ie. DEFAULT NULL).


for example , there is 60 columns , 20 is default null ,  there will be 3 bytes null_maps ,  but how can we map null_maps to 60 columns ?
