In the British Army, officers hold a commission, which confers on them the status and powers of their rank. It is issued in the name of the sovereign, and the holder is issued a document laying out the commission which is signed by the sovereign. Between 1660 and 1871, it was possible to purchase these commissions. For over 200 years, purchase was the traditional method of promotion in the British Army. It was not possible to purchase commissions in the Royal Navy.

The purchase system, as it is often called, was relatively simple. First, a prospective officer would need to purchase a commission for the lowest commissioned rank, which was an ensign in the infantry or a cornet in the cavalry. To do this, there would need to be a vacancy in a given battalion or regiment. To progress through the ranks, this officer would then need to purchase commissions for higher ranks as vacancies appeared either through officers retiring, selling their commissions or transferring to other regiments. This could be done up to the rank of lieutenant colonel, so theoretically - assuming someone had enough money and there were sufficient vacancies - it was possible to attain the command of an infantry battalion or cavalry regiment via financial transaction alone.

The actual economics of the purchase system were relatively simple too. The Crown maintained oversight and reserved the right to approve or deny any purchases. Because of this, an individual was technically buying their commission from (or selling it back to) the government, with the regimental agent (essentially a banker/accountant, responsible for a regiment's financial affairs on behalf of the colonel) acting as a middleman.

The government established set prices for commissions in each rank, and an officer only needed to pay the difference in value between their current commission and their next. It was technically illegal to require an officer to pay more than the set price, but the value of commissions were thought to be greater than the prices established by the government, so non-regulation payments were often sought by those selling their commissions; prospective buyers would pay the regulation sum to the government, and then add a privately-agreed (and illegal) additional payment which was paid directly to the seller via the regimental agent.

Commissions weren't cheap. Richard Holmes gives the example of Edward Cooper Hodge, who commanded the 4th Dragoon Guards during the Crimean War. Hodge did not have to pay for his coronetcy because it was given to him under the patronage of the Duke of York, the Commander-in-Chief of the Forces; if he had paid for it, it would have cost him £840. His lieutenancy cost £350 plus a non-regulation £250; his captaincy was £2,035 plus an additional £1,200; his majority cost £1,350 with a further £1,435; and he paid £1,600 with a non-regulation payment of £1,400 for his lieutenant colonelcy. Some regiments were more expensive than others. Fashionable regiments like the Guards and senior cavalry and line infantry regiments, attracted a premium, whereas newly-raised regiments were easy to purchase into and commissions in them were almost impossible to sell.

Until 1855, the value of the commission was lost if an officer died. Exceptions were made in some cases to allow the officer's surviving family to sell the commission, but this was not the norm. After 1855, the commission could be sold by family members in the event of an officer's death.

Purchase was not the only way to get a commission. As in the case of Edward Cooper Hodge, they could be awarded through the patronage of well-connected family or friends. This was true at all ranks, not just the most junior. Commissions and promotions could also be awarded to reward long service or gallantry. There were also "volunteers", men of middle or upper class who joined and served as private soldiers but messed with the officers and hoped to demonstrate their skill and bravery in order to be awarded a commission. Promotion from the ranks was also not as uncommon as many people believe, especially in wartime.

Promotion could also be gained via seniority. This only applied when vacancies occurred when officers died or were killed. In the case of a death vacancy, each remaining officer moved up a place in regimental seniority without having to purchase. It was technically possible to move all the way from ensign to lieutenant colonel through seniority alone, although this was easier when regiments were serving in especially disease-ridden postings or were regularly engaged in fighting.

The mix of purchase and seniority meant that each vacancy created a chain reaction down the entire unit. If the lieutenant colonel died, the senior major would automatically be promoted into his place and so on down the chain until a vacancy was created at the level of the most junior ensign. If the lieutenant colonel sold his commission, the senior major would be approached first to purchase it. If he did not want it or could not afford it, it was customary to offer the purchase to the next most senior officer and so on down the line. Some officers might club together to raise enough money to persuade a senior officer to sell their commission, thereby setting off a chain of vacancies which would allow them all to benefit.

The combination of purchase and other means of promotion also meant that it was likely an officer would have received at least one promotion without purchase. When it came time to sell the commission, he could therefore benefit from the value of a commission he had not had to buy. This allowed officers to earn money from the system, or at the very least top up the value of the commission they sold to provide a pension for themselves. In many ways, the purchase system shared a lot of characteristics with the modern property market or other similar types of investment.

Despite this seeming like a financial free-for-all, there were rules. Abuses like the selling of commissions to children were made illegal in the early 18th century, although the very young often purchased (or had purchased for them) commissions as ensigns. Rules also obliged officers to offer their commissions for sale only to officers one rank their junior; for example, a major could only sell his commission to a captain, a captain to a lieutenant and so on. In the early 19th century, rules were also set that an officer must serve for two years as an ensign and lieutenant before he could purchase a captaincy, and have served for six years before he could purchase a major's commission. These limits were increased to 3 and 11 years respectively in 1809, and it was also established that a major needed 11 years' service in total before purchasing a lieutenant colonelcy. Many regiments also established a minimum standard of competence even for officers who had purchased their commissions. Holmes cites the example of the 43rd Foot in Spain in 1812, which required a new officer to satisfy the battalion's adjutant that he could properly drill a company before being allowed any command responsibility; it usually took six months of four one-hour drills per day to achieve this level of competence.

Overall, Holmes cites a figure that two-thirds of commissions awarded between 1660 and 1871 were the result of purchase.