An Unassuming Club is a system in the Vienna/Polish Club family, in which 1 shows any of a variety of hands and 1/ shows at least 5 cards. Early versions of Polish Club, and Vienna, and (I think) Breakthrough, limited the 1 opening to less than GF strength and used 2 (Polish and Breakthrough) or 1NT (Vienna) for the really big hands, while using 1 for hands hands in the 19-23-point range and some weaker ones (usually real clubs and/or a balanced range or two). More recent versions of Polish club, and Power System and a few others, use a natural, limited 2 opening and open all the strong hands 1. AUC is in the latter family, but with a weak notrump.
Opening bid structure
Reasons: I don't like nonforcing bids in short suits when I hold a strong hand. Guaranteeing 4 diamonds is useful but guaranteeing 5 seems a bit much. The Precision-style 2 opening is difficult for even great players to handle, as can be seen from several hands from world championship play. Complicated hands should be opened at the 1-level if possible. This system is designed around the ACBL GCC, so the choice in 2-level openings is severely limited.
In fact, this is a playable system with 5-card majors limited to at most 18 points, a 4-card 1 opening limited to at most 18 points, a 2 opening that's infrequent but wonderful when it does come up, and a 1 opening that is complicated but manageable.
Bidding over the 1 opening
The main differences from similar systems in the later auction
The main differences from similar systems in the later auction:
1-1S; 2 shows a minimum opener with 5+ clubs and 4 hearts. 2 here instead shows a stronger hand. Some such trick is necessary to get the 4-card majors out of 2. 1-1; 1H is an artificial strong bid, as in Relay Precision. Instead, 1 shows an 11=18-pt hand with clubs and spades (possibly 4=4=1=4), and 2 shows 5+ clubs and 4 hearts. A 1NT rebid is 15-17, and bids from 2 up are specialized strong rebids.
The auction 1-1; 1NT showing a balanced hand is avoided in Vienna and Polish and Roman, because their balanced hand here is 12-14 or 12-16; instead, with their minimum balanced hand they rebid 1/, possibly a 3-card suit. (Roman rebids 1NT with 15-16, but the principle stands.) Since our minimum balanced hand is 15-17, the 1NT rebid is more attractive and the 1/ rebid (possibly playing in a questionable 4-3 or even 3-3 fit) is less attractive; the opponents probably can't make anything. Playing a bad contract at the 1-level may be OK when it's the opponents' hand, but not when it's our hand.
Anyway, that's the basic structure. Obviously there's a lot more to it.