A branch of dentistry that deals with the treatment and correction of crooked teeth and other irregularities.

Orthodontics has been practiced since ancient times, but the elaborate techniques familiar to us today were introduced only in recent decades. Braces, retainers, and headgear are used to fix such conditions as crowding of the teeth and overbites. According to a 1939 text, “ Speech defects, psychiatric disturbances, personality changes, . . . all are correctable through *orthodontic* measures,” though many adolescents, having endured the embarrassment of rubber bands breaking and even of entangling their braces while kissing, might disagree.

(1) Holding established beliefs, especially in religion. (2) Conforming to established rules or traditions; conventional.

An orthodox religious belief or interpretation is one handed down by a church's founders or leaders. When capitalized, as in *Orthodox Judaism, Orthodox* refers to a branch within a larger religious organization that claims to honor the religion's original or traditional beliefs. The steadfast holding of established beliefs that is seen in religious *orthodoxy* is apparent also in other kinds of orthodox behavior. Orthodox medical treatment, for example, follows the established practices of mainstream medicine. *Unorthodox* thinking is known in business language as “thinking outside the box.”

The correction or prevention of deformities of the skeleton.

Just as an orthodontist corrects crookedness in the teeth, an *orthopedist* corrects crookedness in the skeleton. *Orthopedics* is formed in part from the Greek word for “child,” and many *orthopedic* patients are in fact children. But adults also often have need of orthopedic therapy, as when suffering from a joint disease like arthritis or when recovering from a broken arm or leg.

The spelling of words, especially spelling according to standard usage.

Even as recently as the 19th century, the orthography of the English language was still unsettled. Not until spelling books like Noah Webster's and textbooks like “McGuffey's Readers” came along did uniform spelling become established in the U.S. Before that, there was much *orthographic* variation, even among the more educated. The many people who still have problems with spelling can take heart from Mark Twain, who once remarked, “I don't give a damn for a man that can spell a word only one way.”

Moral integrity.

We associate straightness with honesty, so if we suspect someone is lying we might ask if they're being “straight” with us, and we might call a lawbreaker *crooked* or label him a *crook*. *Rectitude* may sound a little old-fashioned today, but the virtue it represents never really goes out of style.

To set right; remedy.

We rectify something by straightening it out or making it right. We might rectify an injustice by seeing to it that a wrongly accused person is cleared. An error in a financial record can be rectified by replacing an incorrect number with a correct one. If the error is in our tax return, the Internal Revenue Service will be happy to rectify it for us; we might then have to rectify the impression that we were trying to cheat on our taxes.

(1) Moving in or forming a straight line. (2) Having many straight lines.

Rectilinear patterns or constructions are those in which straight lines are strikingly obvious. In geometry, *rectilinear* usually means “perpendicular” ; thus, a rectilinear polygon is a many-sided shape whose angles are all right angles (the footprints of most houses, with their extensions and garages, are good examples). But *rectilinear* is particularly used in physics. Rectilinear motion is motion in which the speed remains constant and the path is a straight line; and rectilinear rays, such as light rays, travel in a straight line.

Something that guides or directs; especially, a general instruction from a high-level body or official.

As the definition states, a directive *directs*. A directive from a school principal might provide guidance about handling holiday celebrations in class. A directive from the Vatican might specify new wording for the Mass in various languages. Even the European Union issues directives to its member countries, which they often ignore.