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A Stretch is a variation of Boos that first appeared in Super Mario Bros. 3. They appear as long bars consisting of Boos that periodically appear, though in other games they are found on solid platforms.
After a long absence, Stretches appear in Mario & Luigi: Rivals Quest. In the overworld they will slighty follow the player, while some Boos appear and pop out of the Stretch and will turn into a normal Boo. The player must battle or quicky ignore them, or they will continuously make more Boos.
Like Pink Boos, pink variations of Stretches appear in the game. They are usually faster and more powerful.
While Stretches do not physically appear in Mario & Luigi: Sage Quest, a portrait of it can be seen in the Conundrum Manor, and the boss Jumb-Boo who is fought in the said location has an attack that involves in it sinking into the ground, turning invisible, and moving under a bro to pop out under them to deal damage.