The modern flush toilet is a comparatively recent invention. Several 19th-century European plumbers, including the infamous Thomas Crapper, worked on perfecting it, but it didn't come into general use in North America until after World War I. A typical toilet has a tank and a bowl, but it's what's inside that separates older toilets from modern ones. Streamlined S-traps, floats and flush valves make modern toilets flush more efficiently while using less water.

The Bowl and Internal Trap

Toilet design incorporates that all-important 18th-century plumbing innovation -- the S-trap. It's molded into the porcelain base that opens to become the business end of the fixture -- the bowl. The trap holds a pool of water in the bowl, and that controls odors while you're using the toilet. Even more important, the water seals sewer gases and harmful microorganisms down in the pipes and keeps the bathroom hygienic. Because of its shape, the trap is the part of the toilet in which clogs are most likely to occur.

The Tank -- Flush and Fill Valves

In early toilets, the tank that stored the water used for flushing was located near the ceiling, and gravity gave the water the momentum to move quickly enough through the bowl to create the suction needed for a flush. On modern toilets, the tank is bolted to the bowl, and a large orifice on the bottom of the tank passes into the bowl. The design allows water to rush quickly into the bowl through holes under the rim. The flush valve includes a flapper that seals the water in the tank until you flush. Also in the tank, the fill valve -- which is connected to the water supply -- comes on when the float falls and opens it.

Contemporary Fill Valves

If you bought your toilet recently, it probably doesn't have the ballcock-style float that you find in the tanks of older ones. Modern fill valves have a cup-style float that slides along the pipe that transports water from the water connection under the tank to the valve aperture, which is always above the water level to eliminate the danger of back-flow from the tank to the water supply. All the working parts on these valves are plastic, and the design allows you to adjust the tank water level with more accuracy.

The Toilet Flange

Although Crapper didn't invent the toilet, he did patent a method for creating an airtight seal, and the toilet flange -- also called the closet flange -- is a version of his invention. A modern flange, often made entirely of non-corroding PVC, fits inside a standard PVC waste pipe. When installing it, the plumber glues it to the waste pipe, then bolts it to the subfloor. The bolts that hold the toilet fit inside tracks on either side of the flange ring.

The Wax Ring and Toilet Bolts

The wax ring forms the actual seal that keeps the water inside the toilet. It's an inexpensive part that you must change each time you remove the toilet for servicing or to clear a clog. It sits on the top of the flange ring and compresses against the bottom of the toilet. The two brass bolts that hold the toilet to the flange are also standard equipment. You should replace these along with the wax ring each time you remove the toilet.