The Trailer Jack Dolly provides a "capture" for your trailer jack or trailer jack wheel. With a pair of the Boat Dolly under each trailer wheel and one with the Trailer Jack Attachment under the jack, your trailer and load can now be moved in any direction. The Trailer Jack Dolly Attachment also preserves the life of your trailer jack wheel, which is often the "weakest link". Easily attaches to our 12" x 16" Boat Dolly, both Standard and Heavy Duty. Boat Dolly not included.