Master and Child contract relationships (aka Master Contracts and Sub-contracts) are easily formed from either the Master or Child record. Child contracts (if any) are displayed in a separate section at the bottom of a contract record. Click on a row in the Child Contracts grid to open that contract.
How to add a child contract to a master contract
Open the Master Contract and scroll to the Child Contracts section.
Click .
Locate and click the check box of the Child Contract from grid and then click .
If the Child record does not yet exist, fill in the Add New Contract fields and then click.
How to select the master contract for a child contract
Open the child record or create a new one. Locate and select the Master Contract from the Master Contract field drop-down (as seen below).
Click 
.