Handedness is the dominance of one hand over the other, or the unequal distribution of fine motor skills between the left and right hands. It refers to the tendency of humans to be more dextrous or skilled with one hand over the other, or sometimes merely the preference of one hand over the other. It is usually used with reference to fine motor skills and the performance of manual tasks, particularly everyday activities such as writing, throwing, etc.

Defined in yet another way, a person�s handedness is the hand used for activities that require a lot of practice and fine motor skills (e.g. writing), or the coordination of large muscle groups to carry out smooth actions (e.g. throwing a ball), both of which are activities employing many neurons in the brain and requiring tightly concentrated and specialized neurological wiring.

Laterality is a similar but slightly broader concept, and refers to the general preference of one side of the body over the other (e.g. handedness, footedness, a preference for using the left or right eye or ear, etc).

There are four or five main types of handedness:

• Right-handedness (or dextrality) - where people are more dextrous with, or primarily use, their right hand when performing manual tasks. This is the most common kind of handedness, covering up to 90% of the world�s population, depending on the definitions used, with perhaps 60% being strongly right-handed (performing ALL activities with the right hand).
• Left-handedness (or sinistrality) - where people are more dextrous with, or primarily use, their left hand when performing manual tasks. Around 10% (possibly more) of the world�s population falls into this category, although only perhaps 3% are strongly left-handed (performing ALL activities with the left hand).
• Mixed-handedness (or cross-dominance) - where people tend to perform different tasks better with different hands, e.g. someone may write with the left hand but throw balls with the right, etc. Depending on the definition, this is a less common (about 5-6% according to some, just 1% according to others), but by no means rare, phenomenon. However, if defined more loosely as someone who does not perform ALL activities with one hand or the other (i.e weak-handedness, as opposed to strong handedness), mixed-handedness is much more common, perhaps in the region of 35%.
• Ambidexterity - where people are able to perform any task equally well with either hand. True ambidexterity is quite rare (some suggest about 1%, others a vanishingly small percentage), and many nominally ambidextrous people actually tend to sway towards one or other dominant hand. Many natural left-handers may learn to become almost ambidextrous out of necessity in a world geared more towards right-handers.
• Ambilevous (or ambisinistrous) - where people have equally poor dexterity (are equally clumsy), with both hands. This is a very rare occurrence, usually resulting from a debilitating physical condition.

It should be noted, of course, that these are not discrete categories, but rather positions on a continuum. For example, very few people are 100% left-handed or 100% ambidextrous. The definitions of right-handed, left-handed, etc, are notoriously difficult to pin down (see the section on Measuring Handedness), and any statistics on the subject necessarily suffer from this ambiguity.

Another kind of handedness that occurs in nature is found in the realm of physics, chemistry and mathematics, where it is usually referred to as chirality, a phenomenon first noted by Louis Pasteur in the late 19th Century. An object is chiral if it is not identical to its mirror image (and cannot be superimposed onto it), in much the same way as a hand is not identical to its mirror image.

The three-dimensional forms of many molecules can have either an L-form (from laero-, or left-handed) or a D-form (from dextro-, or right-handed). In molecular terms, our bodies are made up almost entirely of L-amino acids and D-sugars. The famous DNA molecule normally forms a right-handed spiral (although a rare left-handed variant can occur), notwithstanding the many incorrect left-spiralling image reproductions of DNA helices found in both the scientific and popular media alike. On an even smaller scale, fundamental particles like quarks, electrons and neutrinos also have a chirality or handedness of sorts.

It is far from clear that our dominant right-handedness or our predominantly left-sided heart (or any other aspect of laterality in humans) owes anything at all to this molecular-level or atomic-level chirality, although such a link has been hypothesized.