About seventy years ago, a group of English top players designed the Acol system. Characteristic elements of that system were: 4-card opening bids in all suits and limit raises in the opening suit or in NT. Acol became the most popular system in the Netherlands. The Dutch developed their own version, internationally known as Dutch Acol. Though many variants of the system exist, we confine ourselves to the basic version here, which is more or less the Lingua Franca of Dutch bridge players.
 

Opening bids:

• 1 = 12-19 hcp; 3-card or longer
• 1 = 12-19 hcp; 4-card or longer
• 1 = 12-19 hcp; 4-card or longer
• 1 = 12-19 hcp; 5-card or longer

• 1NT = 15-17 hcp balanced; Stayman and Jacoby transfers
• 2 = game-forcing or NT hand with 23-24 hcp (response of 2 = relay)
• 2 = multi colored: weak 2 (max. 10 hcp; 6-card) in / or semi-forcing in /; responses:
• 2 = non-forcing relay
• 2 = non-forcing relay, support in (invites to 4)
• 2NT = asking bid, responses:
• 3 / = non-minimal weak two in /
• 3 / = minimal weak two in /
• other bids: strong and natural
  
• 2/ = semi-forcing
• 2NT = 20-22 hcp balanced; Stayman and Jacoby transfers
• 3/// = preemptive, max. 10 hcp; 7-card

2 opening bid

2 multi colored opening bid. In that case the

2 opening bid may include NT hands with 25 hcp or more.

• 'rule of twenty': the total number of cards in the two longest suits plus the number of hcp should at least add up to 20; examples: two 5-cards and 10 hcp justifies an opening bid, a 4-3-3-3 distribution requires at least 13 hcp for an opening bid
  
• 'rule of fifteen': in the fourth hand (= after pass-pass-pass), an opening bid is justified if the number of hcp plus the total number of is at least 15, even if the rule of twenty is not satisfied; example: a 5-3-3-2 distribution with a 5-card in and 10 hcp justifies an opening bid of 1  in the fourth hand; a 4-4-3-2 distribution with 4-cards in both and , and 11 hcp justifies an opening bid of 1 in the fourth hand

• raise in opening suit: 2/3 level = 6-9/10-11 hcp, or less with compensating distribution
• 1 of a new suit = 6 hcp or more; 2 of a new suit = 10 hcp or more
• jump shift = 12 hcp or more; 6-card or longer
• 1NT = 6-9 hcp (no support in opening suit; not necessarily balanced)
• 2NT = 10-11 hcp (no support in opening suit; balanced)
• reverse rebid (2 of a new suit, higher than opening suit) = strong
• any jump bid = strong (in a new suit or in NT always game-forcing)

Conventions:

• 4th suit = conventional: partner should respond in NT if he has a stop in that suit or else further describe his hand
• 4NT = Blackwood; responses: 5 = 0 or 3 aces ; 5 = 1 or 4 aces; 5 = 2 aces

Competitive bidding:

• negative doubles up to 3
• jump overcalls are intermediate = about 10-14 hcp; 6-card or longer
• takeout doubles after a preemptive opening

• 1 opening bid = a 5-card (like 1);  this has consequences for the opening bids of 1/ (1 = a 2-card being the most popular choice)
  
• 2/ opening bid = max. 10 hcp; exactly 5-card in the major + 4-card or longer in / (so-called Muiderberg); the 2/ semi-forcing opening bid moves to the strong 2 opening bid