1. In an optional manner
   "this rule is applied optionally"

See also: optional

Antonym: obligatorily

• trunnion
• truss
• truss bridge
• trussed
• trust
• trust account
• trust busting
• trust company
• trust corporation
• trust deed
• trust fund
• trust territory
• trustable
• trustbuster
• trusted
• trustee
• trustee account
• trustee-beneficiary relation
• trusteeship
• Trusteeship Council
• truster
• trustful
• trustfully
• trustfulness
• trustiness