Why did Lincoln say four score and seven instead of eighty seven?

Answer 1

* it was on the 4th of July that Lee retreated after the Battle of Gettysburg (and on that same day, Grant's siege of Vicksburg was finally successful, as that fort surrendered)

* Lincoln saw this timing as significant, esp. given his longtime conviction that the struggle over slavery, and then the Civil War, were rooted in a struggle over the central principle of the Declaration of Independence that "all men are created equal" (which, he insisted, included blacks); in fact, Lincoln immediately goes on in the speech to quote and argue from that line ("dedicated to the proposition that all men are created equal")

* the form of expression echoes a traditional biblical form (in the King James Version), e.g., "threescore years and ten" (in Psalm 90); Lincoln frequently used biblical quotes & allusions (with which many of his countrymen would be familiar) in his speeches, e.g., "a house divided against itself cannot stand" (based on a statement of Jesus in the Gospels); this made the line not only more 'poetic', but added an air of solemnity & authority to it

* this is more speculative, but perhaps Lincoln even intended that people should think of the specific expression in Psalm 90, which refers to the human lifespan. In that case he may have been setting up a key image of the speech - of conception and birth, esp. NEW birth (of freedom) accomplished through the sacrifice/death of those who had fought here. That is to so, through this conflict the nations was in a sense dying & being reborn.

* Article V of the US Constitution had specified that no amendments about importation of slaves could be addressed until 1808, which gave the nation 20 years (one score) after ratification to deal with the issue of slavery. Lincoln may have been deliberately alluding to the fact that our nation had ignored this issue four times over that allotted by our founders.

Answer 2

it was back then, they talked different