Sesquiplanes enjoyed a moment in the sun as designers grappled with the robust structural truss integrity of biplanes versus the lower drag of monoplanes. Sesquiplanes, effectively a “monoplane and a half,” as described by aviation historian Joseph Juptner, used an abbreviated lower wing to enable bracing, while keeping overall span short, and reducing drag below that of a full-bore biplane.
Source: general aviationA monoplane and a half