While holding the alt key down type 132 on the Numbers Keypad. Release the Alt key and ä appears. If you made an error use backspace once. This does not work with the numbers across the top of the keyboard. This method works with many word processors and HTML editors but not with all.

ä 132 Ä 142
ö 148 Ö 153
ü 129 Ü 154
ß 225

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Over 21 years I owned several:
w108
w110
w111
w115
w116
w123
w124
w126
w212