This article is from the Mastiffs Breed FAQ, by mmcbee@access.mountain.net (Mike McBee) with numerous contributions by others.
Clean, quiet, and undemanding. Heaven to a Mastiff is a rug beside
his owner's chair. Mastiffs are naturally clean (except for
slobber), and quick to housebreak. Most adult Mastiffs don't chew
what they shouldn't, and they don't get on the furniture (unless
you let them ;-)).
If you do let your young Mastiff on the sofa, just remember that
they grow FAST, and it is unfair and quite unreasonable to let the
dog learn to enjoy something, and then decide the dog is too big
to get up there any more. Mastiffs have long memories, and are
much easier to train correctly the first time than to retrain to
get rid of bad habits.
 
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