The electoral system of Baden-Württemberg looks a bit different from those of most German states, and resembles the system used for the Bundestag in 1949 more than anything. The voter gets a single vote, given to a candidate in their constituency. In each constituency the election works on simple plurality, after which the votes are tallied statewide combining all candidates put up by each party. A proportional calculation is made for 120 seats, and if a party has gotten more constituency seats than they would be entitled to, the size of the Landtag is increased until proportionality is achieved (since 2013, this is how it works for the Bundestag as well).
The seats given out proportionally, however, are not distributed according to any party list. Instead, they are given out to those constituency candidates who got the best percentage results without winning their seats. These candidates are declared elected for their constituencies as well, obtaining what is called a
Zweitmandat, or second mandate, in the constituency. There's no system for distributing these evenly between constituencies, so a given constituency may have one, two or three representatives purely depending on the statewide results - generally, more closely-contested constituencies will more likely get
Zweitmandate, but there's no hard and fast rule.
The 2016 election was somewhat historic - in 2011, the CDU, in power since 1953 with one brief exception, was defeated amidst debate over the hugely-controversial Stuttgart 21 transport project. The Greens reaped most of the benefits, and a coalition was formed between the Greens and SPD under Green leader Winfried Kretschmann. This proved somewhat popular, and Kretschmann in particular established himself as a Prime Minister with strong centrist to centre-left appeal. In 2016, this incumbency paid dividends, and Kretschmann won the Greens their first-ever plurality on the state level. The SPD collapsed utterly, falling behind the AfD (which picked up traditionally-SPD working-class voters in industrial cities like Mannheim and Pforzheim), and Kretschmann reformed his government as a "grand coalition" of sorts with the CDU.