(1) A quarter of a circle. (2) Any of the four quarters into which something is divided by two lines intersecting at right angles.

This word is used for a traditional instrument, used to make calculations of altitude and traditionally employed by sailors to navigate, which has a piece shaped like a quarter of a circle. A quadrant shower is a shower that fits snugly into a bathroom corner and displays a curved front, making a quadrant shape on the floor. But perhaps *quadrant* is used most often today to name a particular quarter of a city.

A square dance popular in the 18th and 19th century, made up of five or six patterns for four couples.

The quadrille, named for its four couples that form the sides of a square, seems to have begun as a French country dance. In the 18th century it became fashionable among the French nobility; as performed by elegantly dressed aristocrats, it became slow and formal. It crossed over to England and from there to New England, where it turned back into a dance for the common people. It soon evolved into the American square dance, a lively type of dance that employs a “caller” to make sure everyone remembers the steps.

Paralyzed in both arms and both legs.

*Quadriplegia* is the result of injury or illness, almost always affecting the spine. Though a *paraplegic* has lost the use only of his or her legs, *quadriplegics* are paralyzed in all four limbs. Today voice-activated wheelchairs help the quadriplegic get around, and houses can be equipped with similar systems to operate lights and appliances; monkeys have even been trained to assist quadriplegics with everyday tasks. The work of the quadriplegic actor Christopher Reeve has led to remarkable advances in developing new nerve connections, enabling some determined paraplegics and quadriplegics to walk again.

One of four equal groups each containing a quarter of a statistical population.

A quartile is a *quarter* of a specific group that has been tested or evaluated in specific ways. The first quartile is the one that scores highest and the fourth quartile scores lowest. For achievement and proficiency tests, the first quartile is the place to be; for blood pressure or cholesterol, the third quartile is healthier. ## TETR **TETR** comes from the Greek word for “four.” In the immensely popular video game *Tetris*, for example, each of the pieces the game is played with has four segments. But the root usually shows up in long chemical names.

A yellow broad-spectrum antibiotic.

Most chemical names are made up of two or more Greek and Latin roots strung together. Thus, *tetracycline*, with its *cycl-* root from the Greek word for “circle,” means “four-ringed”—that is, “consisting of four fused hydrocarbon rings.” Antibiotics work against bacteria and other tiny organisms (but not viruses); tetracycline, which comes from a kind of soil bacteria, is one of the most used of the antibiotics. “Broad-spectrum” antibiotics work well on numerous organisms; thus, tetracycline has proved effective against acne, chlamydia, cholera, rickets, and various lung and eye infections, among many other conditions.

A solid shape formed by four flat faces.

The simplest tetrahedron is made of four equal-sided triangles: one is used as the base, and the other three are fitted to it and each other to make a pyramid. But the great pyramids of Egypt aren't tetrahedrons: they instead have a square base and four triangular faces, and thus are five-sided rather than four- sided.

A set of four connected literary, artistic, or musical works.

Vivaldi's *Four Seasons* could be called a tetralogy, since it's a set of four violin concertos, one for each season of the year. Eight of Shakespeare's history plays are often grouped into two tetralogies. Wagner's great *Ring of the Nibelung,* an opera tetralogy based on Norse mythology, contains about 18 hours of music. The original tetralogies, however, were sets of four plays by the same author performed together in ancient Greece; the first three were always tragedies, and the last was a wild comedy. Tetralogies were written by such great dramatists as Aeschylus, Sophocles, and Euripedes; unfortunately, none of them have survived in their entirety.

A vertebrate with two pairs of limbs.

The earliest tetrapods, or “four-footed” animals, were mammal-like reptiles that evolved before the rise of the dinosaurs and ranged from mouse-sized to cow-sized. Today the tetrapods include the reptiles, the amphibians, the birds, and the mammals—including humans. Though the fish aren't classified as tetrapods, it's quite possible that our own limbs began as paired fins hundreds of millions of years ago.