A rectangle, sometimes called a straight or banana shape, has a bust and hip that measure close to each other and a waist that is not dramatically narrower than either. The outline runs fairly straight from shoulder to hip.
It shares its bust-to-hip balance with the hourglass exactly. The only thing separating the two is waist definition: near 0.84 of the hip here against near 0.72 for an hourglass. That single figure changes the advice completely, because an hourglass has a waist to follow and a rectangle has one to create.
