Explainer / Tanakh
Psalm 110:1
A Psalm of David. Yehovah says to my lord, Sit at my right hand, Until I make your enemies your footstool.
לְדָוִד מִזְמוֹר נְאֻם יְהוָה לַאדֹנִי שֵׁב לִימִינִי עַד־אָשִׁית אֹיְבֶיךָ הֲדֹם לְרַגְלֶיךָ
Compare with the ASV
A Psalm of David. Jehovah saith unto my Lord, Sit thou at my right hand, Until I make thine enemies thy footstool.
We printed the second word as "my lord," with a lowercase l, where the ASV has "my Lord."
Why
Two different words
The first is the Name, יְהוָה, rendered Yehovah.
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The second is אֲדֹנִי, adoni, "my lord," the ordinary word for a superior: a king (1 Sam 24:8), a master (Gen 24:12), a husband (Gen 18:12), an angel (Josh 5:14).
Adoni is not Adonai
The form אֲדֹנָי, Adonai, is used of God.
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The Masoretic text points this word as adoni. The consonants לאדני are the same for both; the vowels were written by the Masoretes in the early medieval period.
The Septuagint
has τῷ κυρίῳ μου, "to my lord," in agreement with the Masoretic pointing.
The same wording in the New Testament
The verse is quoted in Matt 22:44, Mark 12:36, Luke 20:42 and Acts 2:34, and this version prints "my lord" in all of them, so the quotations read the same as the psalm.