What's the stroke order logic of 臣?

I recently tried to input the stroke order for 臣 into my cell phone's dictionary.  I had a heck of a time because I kept thinking that the first stroke would be verticle (see 1b below) as it is with 口 (see 2 below).  But no, when writing 臣 the furthest left verticle stroke doesn't come until the very end (see 1a).  What is the logic here?  Specifically, why is 1a right and 1b wrong?

 PS: Sorry about the primitive drawing, hehe.