Numbering the Psalms

We have numbered the Psalms according to the Septuagint, with the New King James Version numbering in parentheses.

Why are the Psalms in the Agpeya numbered differently than those in the Holy Bible? For example, Psalm 50 in the Agpeya is Psalm 51 in the Holy Bible.

The psalms are numbered differently in the Agpeya based on the numbering used in the Septuagint (Greek Manuscript). The Septuagint translation is highly regarded by the Orthodox Churches as the oldest and most reliable manuscript of the Holy Bible. The numbering differences begin with Psalms 9 & 10 (NKJV) = Psalm 9 (Septuagint). Because of the similarity in theme and pattern, these psalms were considered one. You may also notice that in the Ninth Hour of the Agpeya, Psalm 114 & 115 = Psalm 116 (NKJV). In the Compline (12th Hour), Psalms 146 & 147 = Psalm 147 (NKJV). Psalms 148-150 are the same in both the Hebrew and Septuagint translations. Psalm 151, which is not acknowledged in the New King James Version (NKJV), commences the Bright Saturday (Apocalypse Night) prayers in the Coptic Orthodox Church.

Taken from His Grace Anba Youssef's "Question and Answer" page