Spanish diacritical marks, also known as tildes, are symbols used to modify the pronunciation of a letter or to indicate stress. They are used in the Spanish language to differentiate between words that are spelled the same but have different meanings. Tildes are the most common of these diacritical marks and consist of two dots placed over a vowel. They are used to indicate a change in the pronunciation of the vowel, such as a change in the sound from short to long.