Most canker sores are round or oval with a white or yellow center and a red border. They form inside your mouth — on or under your tongue, inside your cheeks or lips, at the base of your gums, or on your soft palate. You might notice a tingling or burning sensation a day or two before the sores actually appear. There are several types of canker sores, including minor, major and herpetiform sores. Minor canker sores Minor canker sores are the most common and: 
Are usually small
Are oval shaped with a red edge
Heal without scarring in one to two weeks
 Major canker sores 
Major canker sores are less common and: 
Are larger and deeper than minor canker sores
Are usually round with defined borders, but may have irregular edges when very large
Can be extremely painful
May take up to six weeks to heal and can leave extensive scarring
 Herpetiform canker sores Herpetiform canker sores are uncommon and usually develop later in life, but they're not caused by herpes virus infection. These canker sores: 
Are pinpoint size
Often occur in clusters of 10 to 100 sores, but may merge into one large ulcer
Have irregular edges
Heal without scarring in one to two weeks
