A brown envelope arrives, and inside it is a jury summons. Your name has been chosen at random from the electoral register, and you are being asked to sit as one of 12 people who will decide the outcome of a criminal trial. It is one of the oldest civic duties in England and Wales, and it is not optional. If you receive a summons, you must respond within seven days, and you can be fined up to £1,000 if you do not return the form or turn up for your jury service.

Jury trials in England and Wales take place almost exclusively in the Crown Court, for serious criminal offences. The jury listens to the evidence, hears the judge's directions on the law, and then retires to the deliberation room to reach a verdict. The judge decides the sentence if the defendant is found guilty. The system depends on ordinary people being willing to do it, which is why the law makes attendance compulsory unless the court agrees otherwise.

To be eligible for jury service you must be aged between 18 and 75 on the day you start, have lived in the UK, the Channel Islands or the Isle of Man for at least five years without any gaps since the age of 13, and be registered to vote. Those three conditions come from the Juries Act 1974, which still governs eligibility, summoning, excusal and deferral. Scotland and Northern Ireland run their own jury systems with their own rules, so the figures and procedures in this guide are the England and Wales ones.

Some people cannot serve at all. You are disqualified if you are on bail, if you have received a prison sentence, detention order, suspended sentence or community sentence in the last 10 years, or if you have ever received a prison sentence or detention order of five years or more, including a life sentence. You also cannot serve if you are currently sectioned under the Mental Health Act, or if you lack the mental capacity to serve, for example because you would be unable to understand or make a decision about a case. Even if you are disqualified, you must still respond to the summons within seven days.

Until 2003, whole professions were automatically excused or ineligible, including police officers, barristers, solicitors, judges and members of the clergy. The Criminal Justice Act 2003 swept most of those categories away. Today a serving police officer or a practising barrister can be called and is expected to serve like anyone else. The only route out is to apply for a deferral or excusal on personal grounds, not by virtue of the job.

Jury service usually lasts up to 10 working days. If the trial is likely to last longer than 10 days, jury staff will let you know. If the trial finishes sooner, you may be asked to be a juror on other trials within the same period. You will usually need to be at court from 10am to 5:30pm, Monday to Friday, although times can vary, and you will need to arrive earlier on your first day. Check your summons letter for the exact time.

On your first day, bring your jury summons form or confirmation letter and some identification, such as your passport, photo driving licence or Home Office documents showing your UK immigration status. If you do not have one of those, you can bring any two documents from the official list, which includes a birth certificate, a credit card with three statements and proof of signature, or three utility bills showing your name and address. Allow extra time to go through security, and expect the jury manager to explain your responsibilities and what expenses you can claim.

There is no strict dress code. GOV.UK says you can wear clothes you are comfortable in, such as jeans and a t-shirt, but not very casual clothing such as beachwear, and nothing on your head unless it is for religious reasons. You can bring a mobile phone, tablet or laptop into the court building and use it in the jury assembly area, and most courts have free wifi. You cannot take a phone, laptop or tablet into the deliberation room, and courts have lockers or somewhere safe to store personal items.

The most common misunderstanding about jury service is about pay. You will not be paid for doing jury service. What you can do is claim some money back if your earnings are affected, plus some expenses such as travel. The loss of earnings allowance is capped, and the cap depends on how long your service lasts and how many hours you spend at court each day.

For the first 10 days of jury service, you can claim up to £64.95 a day to help cover loss of earnings and the cost of any care or childcare outside your usual arrangements, if you spend more than four hours at court. If you spend four hours or less at court, the cap is £32.47 a day. If your service lasts longer than 10 working days, the amounts increase: up to £129.91 a day for more than four hours at court, and up to £64.95 a day for four hours or less. Loss of earnings and care costs share a single daily cap, so they are one pot, not two separate allowances.

Food and drink is a separate allowance. You can claim up to £5.71 a day if you spend up to and including 10 hours at court, and up to £12.17 a day if you are there for over 10 hours. Travel is also separate: bus or underground is the cost of the ticket, train is the standard class return fare, a bicycle is 9.6p per mile, and a car or motorcycle is 31.4p per mile. If you carry another juror as a passenger in your car, you can claim an extra 4.2p per mile for one passenger and 3.2p per mile for each additional passenger. A taxi fare can be claimed, but you should ask the court for permission before using a taxi. You can only claim for travel inside England and Wales, and you cannot claim for accommodation.

Your employer must let you have time off work for jury service, but they can ask you to delay it if your absence will have a serious effect on their business. Your employer can choose whether or not to pay you during your service, and many do continue to pay, but there is no legal requirement for them to do so. If they do not pay you, you can claim for loss of earnings from the court, and your employer will need to fill in a loss of earnings form, which you should bring to court on your first day. If you are not allowed to take time off, you can complain to an employment tribunal, and if you are sacked because you did jury service, you may be able to claim unfair dismissal.

Self-employed jurors claim on the same daily caps, but the evidence is different. You need to fill in a self-employed loss of earnings form and include evidence of lost earnings, such as your most recent tax return. If you are not working, the same caps apply to care and childcare costs outside your usual arrangements. To claim care costs, you and the carer need to fill in the care expenses form, and if a family member or friend is looking after your children, they must write a letter saying how many hours they cared for your child. Bring your child's birth certificate or passport to court, or attach a copy to the claim form.

If you get benefits or financial support, show your jury summons to your benefit office or work coach as soon as you get it. You will continue to get financial support and benefits, such as Universal Credit, for the first eight weeks. After that, the court will give you a loss of earnings form to give to your benefit office or work coach.

Make your claim for expenses at the end of your jury service, and no more than 12 months after your jury service started. You will usually be paid seven to 10 working days after submitting your claim form. The court may be able to pay your expenses during the trial if it is likely to last a long time or if you are facing financial hardship, so ask jury staff if that applies to you. GOV.UK also has an online calculator to estimate what you can claim.

You cannot simply refuse a summons because the dates are difficult. If you cannot do jury service on the dates in your summons letter, you can ask to change the date or be excused. You can ask to change the date once, to another date within the next 12 months, and you will need a good reason, such as an operation, an exam, a holiday already booked, being a new parent, or currently living outside England or Wales. When you reply, explain your reasons in detail and suggest three possible dates in the next 12 months that work for you.

Being excused altogether is harder. You can ask to be excused only in exceptional circumstances, for example a serious illness or disability that prevents you from doing jury service, being a full-time carer, being a new parent who will not be able to serve at any other time in the next 12 months, or living outside England or Wales. You can also ask to be excused if you have done jury service in the last two years. You may need to give proof, such as a letter from your doctor. If your request to change the date or be excused is refused, you can appeal by writing to the Jury Central Summoning Bureau with your juror number, name and address, date of birth, the court you were summoned to, and the dates of your service.

If you have a disability or a physical or mental health condition, courts can make reasonable adjustments to help you serve. These can include ramps, access to accessible toilets, a hearing loop, guidance in alternative formats such as large print, audio or braille, and a British Sign Language interpreter. Request the help you need when you reply to your summons. If your service begins in less than a week, contact the court directly to discuss adjustments.

The rules once you are on a jury are strict, and breaking them can be a criminal offence. Do not discuss the trial with anyone until it is finished, except with other jury members in the deliberation room. After the trial you must not talk about what happened in the deliberation room, even with family members, although you can talk about what happened in the courtroom. Do not post comments about the trial on social media, even after the trial has finished: this is contempt of court and you can be fined or sent to prison. You must also not research the case, look up the defendant or witnesses online, visit the scene, or ask someone else to do it for you.

If anyone approaches you about the trial, tell a court officer, or a police officer if you are approached outside court. If you find the trial distressing, speak to court staff, who will give you advice. For emotional support, speak to your GP, or contact the Samaritans, who cannot give advice but can listen.

The short version is this: a summons is a legal obligation, not an invitation. Reply within seven days, turn up unless the court has agreed otherwise, claim what the court allows rather than expecting a wage, and keep everything you hear in the deliberation room to yourself. Done properly, it is two weeks at most out of your life, and it is how the criminal justice system puts ordinary people at the centre of its most serious decisions.