Rule 1.
When a word is prefixed with the definite article, the first consonant of the word is emphasized with a דּגֵשׁ (DDaGeSh). While the definite article receives a פַּתָּח (PPaTTaX).

סֵפֶר	book
הַסֵּפֶר	the book

Rule 2.
However, the following consonants are unable to receive a דּגֵשׁ

• א
• ע
• ר

The definite article then takes up the responsibility of emphasis from such a consonant, by being given a קָמָץ (QaMaTs) instead of a פַּתָּח .

עִיר	city
הָעִיר	the city

Rule 3.
The vowelisation of the definite article is further modified to receive a סֶגוֹל (segol) הֶ instead, if the first consonant of the word is either

• הָ
• חָ
• עָ

having a פַּתָּח vowel.

עָרִים	cities
הֶעָרִים	the cities

Exception to Rule 3.
If the first consonant is pronounced with an accent, the definite article reverts to receiving a פַּתָּח as its vowelisation.