Pedigree Forge usually recognises twins and other multiple births automatically when the children share the same date of birth. The convention on family charts is that multiple births are drawn linked to their parents from a common stem, while other siblings each have their own:
In this example, Kate and Grace have been automatically identified as twins because they share the same birth year (1856).
If no dates of birth have been entered, Pedigree Forge cannot detect the multiple birth automatically:
You can manually set or clear the multiple-birth flag on the Families tab of the Current Person Other Detail dialog (click the … button next to the name fields). Both children must be flagged as multiplets for the shared stem to appear.
The Not multiplet checkbox handles the reverse situation — if two children have approximate dates of birth in the same year and Pedigree Forge incorrectly assumes they are twins, checking this box overrides the automatic detection.