Jamieson, Fausset and Brown on Exodus 12:7

7. take of the blood, and strike it on the two side-posts, &c.--as a sign of safety to those within. The posts must be considered of tents, in which the Israelites generally lived, though some might be in houses. Though the Israelites were sinners as well as the Egyptians, God was pleased to accept the substitution of a lamb--the blood of which, being seen sprinkled on the doorposts, procured them mercy. It was to be on the sideposts and upper doorposts, where it might be looked to, not on the threshold, where it might be trodden under foot. This was an emblem of the blood of sprinkling (He 12:24; 10:29).