Once upon a time, Windows was 16-bit. Each message could carry with it two pieces of data, called WPARAM and LPARAM. The first one was a 16-bit value (“word”), so it was called W. The second one was a 32-bit value (“long”), so it was called L.

You used the W parameter to pass things like handles and integers. You used the L parameter to pass pointers.

When Windows was converted to 32-bit, the WPARAM parameter grew to a 32-bit value as well. So even though the “W” stands for “word”, it isn’t a word any more. (And in 64-bit Windows, both parameters are 64-bit values!)

It is helpful to understand the origin of the terms. If you look at the design of window messages, you will see that if the message takes a pointer, the pointer is usually passed in the LPARAM, whereas if the message takes a handle or an integer, then it is passed in the WPARAM. (And if a message takes both, the integer goes in the WPARAM and the pointer goes in the LPARAM.)

Once you learn this, it makes remembering the parameters for window messages a little easier. Conversely, if a message breaks this rule, then it sort of makes your brain say, “No, that’s not right.”