Three words carry on the chain of blessings:
(1) "Length of days" (see the Psalm 91:16 note);
(2) "Years of life," i. e., of a life worth living (compare Psalm 30:5; Psalm 42:8);
(3) "Peace," tranquility inward and outward, the serenity of life continuing through old age until death. Compare 1-Timothy 4:8.

For length of days - Three eminent blessings are promised here:
1. ארך ימים orech yamim, long days;
2. שנות חיים shenoth chaiyim, years of lives;
3. שלום shalom, prosperity; i.e. health, long life, and abundance.

For length of (a) days, and long life, and peace, shall they add to thee.
(a) Long life is the blessing of God which he gives to his, so far as it is expedient for them.

For length of days, and long life,.... Or, "years of life", or "lives" (f); a long life of usefulness and comfort here, and eternal life hereafter: the law of Moses promised a long life to the observers of it; but the Gospel of Christ brings an immortal life to light and promises to all believers in him that they shall not perish, but shall have everlasting life. "Length of days for ever and ever" was asked by Christ of his Father, Psalm 21:4, and given him, and is in his right hand, Proverbs 3:16; it is in his power to give, and he does give it to all his children, people, and followers, Psalm 21:4;
and peace shall they add to thee; the Gospel is the Gospel of peace, which not only proclaims peace by the blood of Christ; but the doctrines of it, such as justification by the righteousness of Christ, pardon by his blood, and atonement by his sacrifice, are the means of giving and increasing spiritual peace in the hearts of believers; and so are the ordinances of Christ, which in Proverbs 3:17; are called, for that reason, "paths of peace"; see Romans 5:1.
(f) "annos vitarum", Montanus.

length . . . life--often promised as blessings (Psalm 21:4; Psalm 91:16).
peace--includes prosperity (Psalm 125:5).
add--abound to thee.