Prior to July 1915 the British Army comprised of "Regular" soldiers, "Reserve" soldiers, "Territorial" soldiers, or "volunteers".

REGULAR SOLDIERS
A man could present himself at a recruiting office or Regimental Depot, and provided he was between the ages of 19-38, taller than 5ft3in and able to pass the medical and physical requirements of the Army, enlist in the Regiment of his choice. A man would then serve typically 7 years in full service of the colours and a further 5 years on the National Reserve. On completion of his service to the colours, a man could opt for 3 sections of National Reserve:

• (A) an undertaking to rejoin at a time of emergency not requiring a General Mobilisation,
• (B) an requirement to rejoin on General Mobilisation,
• (C) having completed the 5 years of Reserve, an undertaking to a further 4 years on Reserve.
  On completion of his Nation Reserve service, a soldier could also enlist in he Special Reserve.

TERRITORIAL SOLDIERS
Created in 1908, the Territorial Force consisted of locally raised battalions of part time soldiers who trained on evenings, weekends and at summer camps. Men had to be aged between 18-38, taller than 5ft3in and physically fit. They would be required to serve at a time of war, but not necessarily overseas.

SPECIAL RESERVE SOLDIERS
The Special Reserve was effectively a form of the National Reserve, where a volunteer, having signed up for a 6 year period of service, would receive 6 months full time training. This would then be augmented with periods of annual training. After the initial 6 years, a Special Reserve soldier could extend his service by a further 4 years. Special Reserve soldiers were not excluded from overseas service at the time of Gerneral Mobilisation.

VOLUNTEER SOLDIERS
The "volunteers" were the result of the movement of the male population to serve their country due to the commencement of the conflict in August 1914. They answered the call from the Secretary of State for War, Lord Kitchener of Khartoum, and were incoporated into a number of "New Armies", sometimes known as "Kitchener's Armies". The Armies that these men formed were allocated K numbers to signify when they were created.

11th August 1914
Kitchener announces his first "call to arms". The men signed up for 3 years service, or the duration of the war. The initial target of 100,000 men was reached in two weeks, and was used to create 6 Divisions. This new army was referred to as 'K1', and it's battalions were allocated to existing regiments, with the inclusion of the word "Service" in their title to distinguish their origin. The divisions were numbered 9 to 14, and the first units arrived in France in May 1915.

28th August 1914
Kitchener asks the country for another 100,000 men to form a further 6 divisions. These become the 'K2' divisions, numbered 15-20. They first started to arrive in France in May 1915.

The situation in France and Belgium at this time led to a large number of men volunteering, and the excess were formed into 'K3', the third 100,000. Their divisions were numbered 21-26 and they first started to arrive in France from August 1915.

Once K3 had been completed, a fourth army of volunteers, 'K4' was commenced. Initially formed as reserve battalions, for a short period they were organised as divisions, before once again being broken down into Training Reserve Brigades.

10th December 1914
Commencement of the creation of the fifth, or 'K5' army. They tended to be locally raised units, such as the famous "Pals" Battalions. When the K4 units were returned to Reserve Units, K5 became K4, and its divisions were numbered 30-35. Units arrived in France from late 1915 onwards.

March 1915
Recruitment starts for the sixth, or 'K6' army, which became K5 with the disolution of K4. As with K5, the units tended to be locally raised "Pals" Battalions. Divisions were numbered 36-41.

July 1915, National Registration Act
The requirement of everyone aged between 15-65 years to register.

September 1915
Government in receipt of all figures regarding men eligable for service but yet to enlist.

16th October 19161915 - 15th December 1915: The Derby Scheme
Introduced by Lord Derby (Director General of Recruiting), allowing men one last chance to enlist voluntarily. Men could also choose to attest, commiting themselves to service when called upon. Men who volunteered had a choice of unit in which to serve.
Attested men were divided into married & single, and then into 23 'year' "groups" by age between 18 and 41. Groups 1-23 were single men, Groups 24-46 married men

December 1915
Re-organisation of Divisions and Brigades to integrate Regular and New Army battalions, with the idea of bringing experience into the Kitchener Armies.

27th January 1916: The Military Service Act
Conscription introduced for all single British men, or widowers without dependants, aged between 18 (on 5/08/1915) and 41 (on 02/03/1916). These men were then divided into 23 'year' "classes", numbered Classes 1-23. Conscripted men had no choice of unit.

25th January 1916
First groups of Derby men called up, Groups 2-5 (19-22 years old and single). This continued at 2 week intervals, ending with Group 1 on 28/03/1916.

3rd March 1916
First classes of single conscripted men called up, the last class being called on 25/03/1916.

7th March 1916
Derby married men groups called up, the last group being called on 13/06/1916

23rd May 1916
Conscription extended to married men born between 1875 and 1897. These men were then divided into 23 'year' "classes", numbered Classes 24-46.

3rd June 1916
Remaining men (conscripted married) called up, ending 24/06/1916.