Name Zuyder Sea (Dutch: Zuyder Zee or Zuiderzee) was abolished 1932. That year it was closed-off with a dam and renamed 'Lake IJsselmeer' (pronounced: 'eiselmeer').

The Zuyder Sea was an 'inland sea' with salt water and high and low tide, thanks to open connection with the North Sea and Atlantic Ocean. Fishery was mainly on herring, shrimps & anchovies. After it was closed-off in the north by a safety dyke in 1932 (red line on above map), the water turned fresh within a few years. Fish changed to species as pike, fresh water perch and eel. The windy caged sea became shallow (2-6m) with rough waves and cold, untransparent murky grey-green water, improving only recently.

During WW2 1939-1945, years after the name change, most still named the basin The Zuyder Sea, especially Allied aircrew who flew almost daily over it to Germany. Some knew there had been a name change and referred to it as 'The Old Zuyder Sea', with 'Old' meaning 'Former'. Maps showed old name Zuyder Sea until deep in the 1970's.


Phonetically the correct pronunciation is 'eiselmeer'.  As in 'eisenhower' or 'eiffel tower'.

It is a name. In writing it is with capital. In Dutch this is  IJ   and spelling is with double s. 

The Dutch IJ explained. 
To write down the 'ei'-sound, the Dutch have three options:

- the 'short ei'. Written as ei            This is a two letter combination. In capital (large letter), only the first letter e is written in upper case. Correct: Eisenhower.
- the 'long ei'. Written as ij               This is also a letter combination, but considered here as one letter. In capital, the entire  ij  is therefore written in upper case: IJsselmeer.
- the i-grec/Ypsilon. Written as y     In capital as Y.

It is very common to see name of the Lake in all three forms (Eisselmeer, IJsselmeer and/or Ysselmeer). However, only correct form is with IJ. Because it is a name and Dutch consider the i+j combination used for sound 'ei' as one letter, it means that upper case (big letters) must be used for the whole i+j combination, therefore: IJsselmeer, not ijsselmeer or Ijsselmeer. The ij is written in lower case (small) letters when not a name, or in the middle of a word, as in ijzer ('eizer' = iron) or city Harderwijk ('Harderweik').

 Correct: IJsselmeer

 Not correct: Eiselmeer, Eisselmeer, Yselmeer, Ysselmeer, Ijsselmeer, Isselmeer, ijselmeer, Aiselmer, IJesselmeer, etc.