Hi
Best bet would be not to use them. You should not really be using a table
to view/edit/delete/etc data.
Use a form / Subform or a query for this
But if you (for some strange reason) really want to use a subdata sheet then
the Master field is the ID of the table you are looking at and the child
field is the ID of the other table (the sub data sheet).
This said you don't have to use the ID's you can use anything that links the
tables
So you could show all people name John or everyone with the same Zip/Post
code etc, etc.
But again this is NOT a good idea and it will go wrong in the end ?, so best
to use a query or a form instead of using the table to type "stuff" into.
HTH