Knife attacks have been front-page news recently following the killing of three 17-year-olds in the UK in less than a week.
Jodie Chesney was killed in an east London park as she played music with friends; Yousef Ghaleb Makki was stabbed to death in the village of Hale Barns near Altrincham; and Ayub Hassan was found with stab wounds to the chest in Lanfrey Place, West Kensington.
Some are now calling for tougher sentences for illegally possessing a knife. But what are the existing punishments?
The sentencing rules
In England and Wales, there are minimum custodial sentences for anyone aged 16 or over caught with a knife in the following circumstances:
- Threatening: They are convicted of using the knife to threaten another person where that person is at immediate risk of serious physical harm
- Repeat offenders: They are convicted of carrying a knife in a public place or on school premises, and they have at least one previous "relevant conviction" of possession a weapon or threatening people with a weapon
In these cases, offenders aged 18 or over would be sentenced to a minimum six months' custodial sentence and a maximum 4 years.
For those aged 16 or 17 the minimum sentence is a detention and training order of at least four months.
In Northern Ireland, the minimum and maximum custodial sentences for these offences are the same.
In Scotland, the minimum custodial penalties apply - with new sentencing guidelines increasing the longest jail term for repeat offenders from four to five years.
However, in all cases judges can choose not to impose the minimum sentence if they believe it would be unjust.
How many people actually go to prison for carrying a knife?
Official figures, published by the Ministry of Justice, show courts in England and Wales dealt with 21,101 knife cases in the year ending June 2018.
This was the highest number of offences since the year ending June 2010, when there were 22,688 cases.
Sentences for all kinds of violent crime have been getting tougher, particularly for knife crime.
Public anxiety about stabbings, legislative changes and firmer guidance for judges and magistrates since 2015 have led to stiffer sentences, although offenders under 18 are still more likely to be cautioned than locked up.
In the year ending June 2018, 36% (7,649) of those convicted were jailed - compared with 23% (6,212) in 2009.
Of those sent to prison for possessing a knife, most - 82% - serve at least three months behind bars. This proportion has risen from 51% from 10 years ago.
The average prison term for those jailed has also gone up from almost five months to well over eight months, over the last 10 years.
Over the same period, there's been a steady decline in the use of community sentences and a sharp drop in cautions, from 33% (9,369) to 11% (2,352).
In Scotland, where knife crime is treated as a public health issue, convictions for carrying knives have remained relatively stable.
In the 2017/18 financial year, 941 knife possession convictions were handed down, according to official government statistics.
Although this marks a small increase on the previous year, the number is around half that seen in 2008/09.
The proportion of custodial sentences given for knife possession was 38% in Scotland in 2017/18 - a level similar to that seen at the start of the last decade.
But as in England and Wales, the lengths of sentences in Scotland for knife possession have increased in recent years.
When do police decide to issue cautions?
Official advice to police in England and Wales tells them there is "an expectation" to prosecute all those who illegally carry and use knives.
The College of Policing guidelines are that 16 and 17 year-olds are no exception to this and should always be charged unless there are exceptional circumstances.
However, children under 16 caught with a knife with no aggravating factors are likely to be given a caution or conditional caution.
The guidance says no more than one caution should be issued - so anyone caught with a knife for a second time should expect to be punished more severely.
However, police do still have some leeway, with the guidance adding "discretion does exist" but only "if the circumstances justify" it.
What about people caught with a gun or acid?
In England, Wales and Northern Ireland, the minimum sentence for possessing a gun is five years in prison for an offender aged 18 or over at the time of the offence and three years for those aged under 18.
In Scotland, those aged 21 or over can face five years in prison with those aged under 21 receiving up to three years.
Following public concern about a spate of high-profile acid attacks, there is new legislation, awaiting approval by the House of Lords, which would make it illegal to possess a corrosive substance in a public place without a "good reason". In Scotland a "reasonable excuse" could also be used as a defence.
The minimum sentence for offenders who already have at least one "relevant conviction" is six months in prison for adults or a four-month detention and training order for 16 and 17-year-olds.
There will be no minimum sentence first-time offenders but the maximum would be a year in jail.