I copied this from a web site.
You must have two separate means of attachment between your vehicle and the trailer. Safety chains should be crossed under the tongue to prevent the tongue from dropping to the road should the primary hitch accidentally disconnect. It is recommended that chain hooks have latches or devices that prevent accidental disconnect. The breaking strength of each chain should equal the gross
weight of the towed trailer. (See: Farm Vehicles and Equipment and The Highway Traffic Act, Appendix A - chain grades charts.) Chains are required for goose neck type trailers that utilize a ball and socket type hitch. Fifth-wheel type hitches that have safety latches do not require safety chains.