The tone marks วรรณยุกต์ [wan-- na/ juk/] characterize the stress of the syllable. They are placed on the initial consonant of the syllable. In double initial consonants they are placed on the second consonant. Is a vowel sign above the consonant already, then the tone mark is written over the vowel sign.

* Depending on the consonant class of the initial consonant
** Valid only in combination with initial consonants of Class 1 (Mid consonants)

Unfortunately the tone marks are only one of several characteristics that determine the final stress. In a separate article of the Tone rules the full criteria are described. At this point it is sufficient for the time being, to get to know the characters themselves and their fundamental importance.