This may be a stupid quetion but...

[iamandrew248]

On most jazz basses i see, right below the strings on the pickguard, theres a slanted bar. Whats the purpose of this?
I have a kramer: focus and mine has a bar ABOVE the string, and i just use it as a place to put my thumb, why are some on the opposite side?

 

[96tbird]

For thumb playing, you rest your fingers on it. Tug bar. I guess Leo F was a thumb player. A lot of the early electric players played thumbstyle.

 

[Roy Vogt]

Later basses had the bar above the strings for use as a thumb rest, but they did indeed start out as finger rests below the strings so you could pick with your thumb.

 

[JimK]

I guess Leo F was a thumb player.

...was Fender a player?
IIRC, Chuck Rainey, in an old '70s Guitar Player, complained about some issues with the Fender bass. One of them was the "thumb rest" that everyone takes off because it's in the way. He also complained about the strings (everyone takes those off)...and, strangely, he also mentioned the pickup cover.
IIRC, he sez everyone removes that, too (because that is the best place to play the strings)...yet, he sez he anchors on it.
Marcus Miller, Will Lee, & Neil Jason (in some videos) are also those that kept the cover in-place.

A lot of the early electric players played thumbstyle.

Not sure about that.
Most early players were either URB players...who used their URB techniques, fingering, etc.
...or guitarists (who probably opted for pick playing).

IIRC, Fender designed the Jazz Bass (thinner neck, 2 pickups, less pedestrian design than the P, etc) for those guitarists that were to double on electric bass.


Over the years, every non-musician/player that picked up my bass...always held it "thumb-style".

 

[kevteop]

Over the years, every non-musician/player that every picked up my bass...always held it "thumb-style".

It's a common sight on promo shots for manufactured bands too.

 

[JimK]

...at least Danny Partridge strummed.

 

[snyderz]

Caught a Sting show a couple weeks ago, and he thumbed all night. Just like me.

 

[96tbird]

I didn't mean Leo was a bass player beyond designing a bass, but for some reason it occurred to him that a tug bar would be good so he put one on. He had some reason to. It's not a stretch to guess as he was developing the P, he personally would plug in and thumb it. Likely his intention was that kids would pick it up with no urb experience and that's how they would play, it was comfortable and lots did.