NASHVILLE, Tenn. — Five execution methods are legal in various places in the United States: injection, electrocution, gas, firing squad and hanging.

Here is a closer look at each:

LETHAL INJECTION

First adopted in 1977 in Oklahoma, lethal injection has become the method of choice in all states that still carry out executions.

Generally, inmates are strapped to a gurney while needles are inserted into the veins and the drugs are pumped in. This method is often seen as the most humane of the five because the inmates are supposed to be sedated before they die. Inmates, though, have been known to writhe and talk during poorly carried out injections.

According to Deborah Denno, until 2009, all states used a three-drug protocol that included a sedative, a paralytic and then the final, fatal drug to stop the heart. Because of drug shortages and legal challenges that claimed the paralytic drug could mask an inmate’s suffering, states are now experimenting with several different protocols.

ELECTROCUTION

New York developed electrocution as an alternative to hanging – which was often a gruesome public spectacle – and executed the first inmate by electric chair in 1890.

Prisoners generally are strapped into a chair with electrodes placed on their heads and legs. Saline-soaked sponges are placed between the skin and the electrodes to aid conductivity.

Denno said the voltage, the number of jolts and the length of time they are administered vary from state to state. Executioners usually give more than one jolt of electricity, to make sure the inmate is dead.

GAS CHAMBER

Nevada developed the gas chamber in the 1920s as an attempt at a humane method of execution, but Denno said it had “horrific problems” from the start.

Inmates are strapped into a chair and the chamber is filled with cyanide gas, which kills by asphyxiation. The inmates are fully awake and conscious as they suffocate, Denno said.

FIRING SQUAD

This method has been used as recently as 2010 in Utah at the request of a condemned man.

Denno said the prisoner is strapped to a chair, as in electrocution and the gas chamber. A cloth target is placed over prisoner’s the heart. Several shooters are given real bullets but one or more are given blanks. Assuming the shooters hit their target, the heart ruptures and the prisoner dies quickly from blood loss.

HANGING

Before 1890, hanging was the principal method of execution across the country. The prisoner stands over a trap door while a noose is placed around the person’s neck, and then the trap door is opened and the prisoner falls. By design, the fall breaks the prisoner’s neck and kills him or her, but Denno said that has often not been the case. In some cases, prisoners have been decapitated from the fall. In other cases, they have strangled over the course of several minutes.