Shield

A defensive shield, commonly referred to as simply a shield and sometimes as a Holtzman shield, was a protective energy field that could surround the person who wore it.

Technical Information
Shields were produced by a Holtzman generator, the field deriving from Phase One of the suspensor-nullification effect. Shields can be calibrated to permit the passage of matter below given speeds. This is vital in personal defense shields, as one would suffocate within a shield that did not admit atmospheric gasses. Depending on the shield's setting, the object's speed while passing through the shield would range from six to nine centimeters per second. A shield could also be set to cover either the left or right side of a person if the specific need for it arose.

Shields used to protect installations can, and usually do, have far lower penetration velocities, as life support technologies can be used to recycle atmosphere while the shield is active.

However, if a lasgun beam hit a Holtzman field, it would result in sub-atomic fusion and a nuclear explosion. The center of this blast was determined by random chance; sometimes it would originate within the shield, sometimes within the laser weapon, sometimes both.

History
With the widespread use of shields, anyone of even minimal importance wore a body shield to protect against criminals, assassins, and accidents. Such a practice made the use of projectile weapons and thrown blades partly obsolete. The only effective combat method was the deft use and careful precision of a handheld dagger, if moved slowly enough. New styles of fencing and knife fighting were developed to take advantage of this one small vulnerability.

By the time of Muad'dib, when thinking machines had long ceased to be a threat, the shield had been adopted for use in personal defense. These shields were form-fitting energy fields which permitted penetration only by objects that moved below a pre-set velocity. As one would be unable to breathe within a shield that did not permit atmospheric gases to penetrate it, man-portable shields had a relatively high penetration velocity, approximately six to ten centimeters per second. However, shields for ships and planetary installations could and often did have extremely low penetration velocities, as artificial life support technologies could be utilized while the shield was active.

Thus, using lasguns in a shielded environment resulted in military and environmental catastrophe, though at least one commander (Duncan Idaho) has used this phenomenon deliberately as a discouragement to his enemies.

On Arrakis, a shield never lasted long because of the planet's conditions. A shield could only remain active for short periods because its harmonic vibrations would attract a sandworm. Unlike a sandworm attracted by a thumper or other means, a sandworm attracted by a shield would be even more dangerous than normal, as something specific in Holtzman energy infuriates them.

Behind the Scenes
The Holtzman Shield is a potent literary device: it makes some directed-energy weaponry impossible against any worthwhile opponent, and also proves traditional projectile-based firearms and missiles ineffective, adding to the feudal atmosphere, and enforces the usage of mêlée weaponry despite other more advanced technology.

Half
Although popular representation in the film showed full-body coverage with the fields, the books also describe a version which does not entirely cover the body, either stylized "half-shield" with a dash or "half shield" with a space:
 * A small, humming half-shield appeared, a rectangular blur in the air that adjusted to its wearer's movement, swinging to protect vulnerable areas. (Hunters of Dune page 78)
 * Duncan parried upward, but the teenage Bashar reversed his feint and turned it into a real attack, punching the blade against the half shield. (Hunters of Dune page 79)

This is introduced as a rare and ancient Ginaz discipline which Duncan Idaho trains the rejuvenated Bashar Miles Teg to use. Their use also re-appears in the prequel trilogy where he trains against Duke Leto who is using a half shield:
 * Leto spun to cover his vulnerable spots with a shimmering half shield. (House Corrino page 259)
 * Duncan jabbed with his knives, dancing on the fringe of the half shield's protection, but Leto deftly parried with short sword and dagger. (House Corrino page 260)
 * He switched off his half shield, and the Swordmaster proudly sheathed his two blades, then helped the Duke to his feet. (House Corrino page 261)

Aside from these three, it is unclear who else uses them, although it is implied that many Swordmasters may know how to.

The Half shield originated in the first novel, "Dune" where Feyd Harkonnen (wearing a full shield) fought a slave gladiator who used a half shield, which was seen as a disadvantage.