This section is from the book "A Practical Treatise On The Joints Made And Used By Builders", by Wyvill J. Christy. Also available from Amazon: Practical treatise on the joints made and used by builders.
Union Joint is a mode of uniting the ends of pipes by-means of a coupling, which always consists of not less than three pieces, two of which are usually either screwed or soldered to the ends of the piping about to be united, whilst the third piece, which is a 4, 6, or 8-sided cap or nut, screws them both up tightly together, a leather washer being always interposed between them. A variety has already been described under Barrel Union, which embodies the principle of all those most in vogue for pipes. Brass and iron unions are either screwed for iron pipes or tinned for lead at both ends, or one end is treated each way. Union T's, nipple and ferrule unions, are similarly prepared, the former having a cap and lining to two out of its three branches or arms, the third being directly screwed or soldered to one of the pipes, whilst the two linings are attached to the remaining two pipes, the whole three-way junction being connected up and completed by means of the caps. A nipple union is sometimes screwed into an iron service instead of a T.
The cup and ball and the universal swivel are varieties of this joint, for both of which, however, flexible tubing is sometimes found a convenient substitute when of impervious texture, though it is hardly ever reliable for the transmission of gas for any length of time.
This is another name for the hydraulic joint.