While the euphemism is a catch-all term that also includes small bombs made with home-made explosives, in Iraq it more often refers to bombs built by insurgents using military-grade ammunition.

"They are unique in nature because the IED builder has had to improvise with the materials at hand," according to a description by GlobalSecurity.org, a Washington-based group that follows military issues.

The US-led force that invaded Iraq in March 2003 toppled the regime of Saddam Hussein with lightning speed, but did not assign enough soldiers to guard abandoned Iraqi weapons and ammunition depots.

The sites were widely looted. Some of the depots remained unprotected for months until news reports of the wholescale pilfering shamed US officials into action.

As a result, Iraqi insurgents have access to hand grenades, land mines and mortar rounds, as well as large artillery shells and "dumb" airplane bombs powerful enough to level a big building.

In the early months of the occupation, rocket-propelled grenades were the weapon of choice for the resistance but the stocks appear to have run down.

According to the Iraq Coalition Casualty Count, which keeps close track of US casualties, more than 25 percent of US military personnel killed in Iraq have been killed by IED blasts.

Methods to trigger the roadside bomb quickly grew in sophistication.

Insurgents began to set them off through detonators triggered by mobile calls. The US military responded by electronically disrupting the signal.

When insurgents moved to light beams -- the kind available in hardware stores -- the military responded by welding metal arms in the front of their vehicles with a vertical bar to disrupt the beam.

But there is still no way to stop a bomb triggered by a charge sent through a hidden wire.

"For every advance we make they make another one," said Sergeant First Class Joseph Barker, an infantry soldier deployed in southern Baghdad. "They're getting better and there is nothing we can do about it."

The military ordered armor plates welded onto trucks and Humvees already in the field, even though the vehicles were not always designed to bear the extra weight.

Army Brigadier General Yves Fontaine, in charge of logistics in Iraq, said in mid-August that roadside bomb attacks on US supply convoys had doubled over the past year, but casualties had dropped because of the extra armor.

Casualties per bomb are indeed down, but the massive increase in the number of bombs attacks means that, overall, the death toll has never been higher.

The number of soldiers killed in IED blasts averaged 11 a month in 2003, 16 in 2004, and nearly double that figure -- 30 -- during the first 10 months of 2005, according to the Coalition Casualty Count.

In recent months, insurgents have increasingly been using shaped charges, which penetrate armor by focusing explosive power in a single direction and firing a metal projectile into the target at high speed.

Shaped charges are "effective against all types of armored vehicles", said Charles Heyman, a senior defense analyst with Jane's Information Group in London.

"Nearly all of the IEDs we'll see from now on will be of this pattern," he said in a telephone interview.

The US military issues almost daily statements on how soldiers find and disable roadside bombs.

On Monday, they said US soldiers in Baghdad had disabled an IED consisting of two 155mm rounds in a fuel can the previous evening. Iraqi police had found another IED "which consisted of a 120mm shell with TNT and small bottles of gas with nails."

Insurgents often add a sugar and gasoline mix that acts like napalm in sticking to a victim's body and burning.

Sergeant Frank Purcell, a Third Infantry Division soldier deployed in Baghdad, was simple in his asessement. "There's no good defense against them," he said.

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