The Grenadier Guards is an infantry

Infantry

Infantrymen are soldiers who are specifically trained for the role of fighting on foot to engage the enemy face to face and have historically borne the brunt of the casualties of combat in wars. As the oldest branch of combat arms, they are the backbone of armies...

 regiment

Regiment

A regiment is a major tactical military unit, composed of variable numbers of batteries, squadrons or battalions, commanded by a colonel or lieutenant colonel...

 of the British Army

British Army

The British Army is the land warfare branch of Her Majesty's Armed Forces in the United Kingdom. It came into being with the unification of the Kingdom of England and Scotland into the Kingdom of Great Britain in 1707. The new British Army incorporated Regiments that had already existed in England...

. It is the most senior regiment of the Guards Division

Guards Division

The Guards Division is an administrative unit of the British Army responsible for the administration of the regiments of Foot Guards and the London Regiment.-Introduction:...

 and, as such, is the most senior regiment of infantry. It is not, however, the most senior regiment of the Army, this position being attributed to the Life Guards

Life Guards (British Army)

The Life Guards is the senior regiment of the British Army and with the Blues and Royals, they make up the Household Cavalry.They originated in the four troops of Horse Guards raised by Charles II around the time of his restoration, plus two troops of Horse Grenadier Guards which were raised some...

. Although the Coldstream Guards

Coldstream Guards

Her Majesty's Coldstream Regiment of Foot Guards, also known officially as the Coldstream Guards , is a regiment of the British Army, part of the Guards Division or Household Division....

 was formed before the Grenadier Guards, that regiment is ranked after the Grenadiers in seniority as it was a regiment of the New Model Army

New Model Army

The New Model Army of England was formed in 1645 by the Parliamentarians in the English Civil War, and was disbanded in 1660 after the Restoration...

.

The grouping of buttons on the tunic is a common way to distinguish between the regiments of Foot Guards.