Cross-dominance

Cross-dominance, also known as mixed-handedness, is a motor skill manifestation where a person favors one hand for some tasks and the other hand for others, while not necessarily being ambidextrous (two-handed), such as someone that writes with a left hand and grabs with the right.

It can also refer to mixed laterality, which refers to a person favoring eyes, ears, feet, or hands on opposite sides of the body. Cross-dominance can often be a problem when shooting or in activities that require aim.

Ambidexterity
One of the most famous varieties of cross-dominance is ambidexterity, (i.e., being equally adept with each hand, or, to a limited degree, foot). The word "ambidextrous" is derived from the Latin roots ambi, meaning "both," and dexter, meaning "right" (as opposed to left) or favorable. Thus, "ambidextrous" is literally "right on both sides".

Although ambidexterity is rare at birth, it can be learned. The key in learning is to start paying attention to minor tasks and performing them with one's opposite hand daily. While difficult at first, minor tasks like brushing teeth, opening doors, and eating will become steadily easier if a person keeps at it. Learning to write or throw with both hands is far harder, but with patience and practice, it is feasible for anyone to become proficient with both hands.

In modern times, it is more common to find people considered ambidextrous who were originally left handed, and learned to be ambidextrous either deliberately or during childhood in institutions such as schools where right-handed habits are often emphasized. Also, since many everyday devices are designed to be only ergonomic for right handed people, many left handed people have no choice but to use the device with the right hand (a good example is a can opener). As a result, left handed people are much more likely to develop motor skills in their non-dominant hand than right handed people (who are not subjected to left-favouring devices). Ambidexterity is often encouraged in activities requiring a great deal of skill in both hands, such as juggling, swimming, percussion or keyboard music, word processing, surgery, and combat.

Popular Culture
In baseball, three 19th-century pitchers are known to have thrown with both hands: Tony Mullane in 1882 and again in 1893, Elton Chamberlain in 1888, and Larry Corcoran in 1884. Greg A. Harris is the only major league pitcher in the modern era to pitch with both his left and his right arm. A natural righty, by 1986 he could throw well enough with his left hand that he felt capable of pitching with either hand in a game. Harris wasn't allowed to throw lefty in a regular-season game until September 28, 1995, the next-to-last game of his career. Against the Cincinnati Reds in the ninth inning, Harris (then a member of the Montreal Expos) retired Reggie Sanders pitching right-handed, then switched to his left hand for the next two hitters, Hal Morris and Ed Taubensee, who both batted left-handed. Harris walked Morris but got Taubensee to ground out. He then went back to his right hand to retire Bret Boone to end the inning. One Division I NCAA pitcher, Pat Venditte of the Creighton Bluejays, regularly pitches with both arms.

In Snooker, Ronnie O'Sullivan is unique amongst the current ranks of top snooker professionals, in that he is able to play to world standard with his left hand. While he lacks power in his left arm, his ability to alternate hands allows him to take shots that would otherwise require awkward cueing. When he first displayed this ability in the 1996 World Championship against Alain Robidoux, the Canadian accused him of disrespect. O'Sullivan responded that he played better with his left hand than Robidoux could with his right. O'Sullivan was summoned to a disciplinary hearing in response to Robidoux's formal complaint, where he had to prove that he could play to a high level with his left hand. He played three frames of snooker against former world championship runner-up Rex Williams, winning all three. The charge of bringing the game into disrepute was subsequently dropped.

In golf, Mac O'Grady was a touring pro who played right-handed, yet could play "scratch" (no handicap) golf left-handed. He lobbied the USGA for years to be certified as an amateur "lefty" and a pro "righty" to no avail. 

In ice hockey, Gordie Howe used a hockey stick with an uncurved blade, so he could use either hand.

In music, Chris Martin lead singer, pianist and occasional rhythm guitarist of the band Coldplay writes with his left hand, plays the French guitar the orthodox way and draws with his right hand.

Etymology
In English, the term ambidexter was originally used in a legal sense of jurors who accepted bribes from both parties for their verdict. Jurors found guilty of such bribery had to forfeit decies tantum, ten times as much as they received.