The proper county or geographic location within California where a lawsuit or petition concerning a trust should be filed and heard. Under California Probate Code § 17005, the primary venue for proceedings involving the internal affairs of most trusts (including petitions for approval of accounts, claims for breach of trust, actions contesting trust validity, and petitions for modification or termination) is generally the county where the principal place of administration of the trust is located. The principal place of administration is usually where the trustee manages the trust’s day-to-day activities or, if none, the trustee’s residence or main office (Cal. Prob. Code § 17002). Different rules may apply to certain testamentary trusts under continuing court jurisdiction.