The written Hebrew language in the period of the Old Testament consisted of consonants only. Later, beginning around 500 years after Christ, when the daily use of the language was diminishing, the scribes developed a system to mark the vowels so as not to lose the sound of the language. They invented symbols for vowels and placed them mostly below the consonants—with a few above—in order not to change the spacing of the consonants nor the length of the roll upon which the texts were written. For example here are the first three words of the Old Testament with the vowels in red:
At the same time, because of concern for the third commandment, the Jews had become reluctant to pronounce the personal name of God, Yahweh, out loud. They believed that the easiest way to avoid violating the commandment was to not pronounce The Name. They did not understand that this commandment had nothing to do with the pronunciation of the name but rather the attitude of the heart. Reverence for the name of God was necessary because His name represents His character. Because of their legalistic attitude, they used the special term Adonai, derived from the title of respect, Adon, or Lord, but in the plural with the first person common singular suffix and a special vowel mark. This title was only used for God. The use of the plural, as in the case with Elohim, was probably intended to be a plural of intensity in order to emphasize that God was exalted and ineffable. Eventually, in the text of the biblical documents, they replaced the vowels of Yahweh with those of Adonai (see the illustration below). This substitution indicated to the reader that he needed to pronounce Adonai in place of Adonai. This combined form—the consonants of Yahweh and the vowels of Adonai—was never pronounced by the Jews. It was simply a visual signal.
Hundreds of years passed, and the first western translators of the Hebrew language, mostly Germans, were not aware of this substitution. Because of this, in their work, the mixture of the consonants of Yahweh and the vowels of Adonai produced the deformation with three syllables, Jehovah. The J is there because in German that is pronounced like the first letter in the Hebrew Yahweh. Later translators came to understand the combination of consonants from one word and vowels from another and, therefore, the error of Jehovah. But, by that time, the form had become traditional and no one wanted to change it.
Today the modern versions do not generally use the form Jehovah. They typically use the English title Lord. But they use the capital letter L and then small caps, producing the form, LORD, thus signaling the name Yahweh. For Adonai the versions generally translate Lord when it refers to deity, or sometimes lord if it is for a man.
In the illustration note in particular the vowels—the points and symbols that are located below or above the consonants.