The absolute ultimate feminine
honor --- a sign of trust and permissiveness that's actually
even greater than her allowing you access "down there" --- namely, a chick's giving you a copy of the key to her house or apartment, so that you can just gleefully waltz right on in and "get it on hot 'n'
heavy" with her anytime you desire.
Fifth base is actually kind of a "mixed blessings" spot to reach on da ol' playin'-field... yes, it is
indeed a wonderful thing if a nice
lady entrusts you with the "magical strip of brass", but
like all situations like this, "with great power comes great responsibility"... you'll want to "guard that key with your life" --- securely tether it to a sturdy key-ring and/or chain, keep it hidden or locked up when you're at
home, and never let it out of your sight when you're traveling... there are
far too many less-than-honorable individuals out there who might wanna access said key for equally-less-than-honorable purposes!