I have a new bike and old understanding of hub and axle standards. I've found that I have a 15mm x 100mm front axle. Easy enough. In the rear, though, I'm not quite sure what I have. The axle is part of the hub, and appears to measure 9mm in diameter. I removed the QR, covered one end of the axle, and used an old spoke to determine the end-to-end width of the axle. It appears to be exactly 145mm. Is 9mm x 145mm actually a spec or am I missing something? 15x100 front and 9x145 rear? I've never heard of that. Should I not count the overlap of the axle with the open dropouts? I think that would make it 135mm, but I'm wondering because I find no wheelsets in that particular combination, making me think I'm doing something wrong. What am I missing?

more likely a 10mm rear axle... and the rear frame between the dropouts should probably be 142mm, not 145mm... standard MTB is 135mm, 142mm, and a few 150's for DH bikes...... and most 142 and 150 hubs use a 12mm axle...

check the inner diameter of the dropouts.... someone might have swapped out the correct wheel for a lesser unit, etc...

A QR axle tends to be 11mm longer than the drop out spacing, so you likely have 135mm spacing w/10mm axle.
What was/is "standard" hybrid/mb spacing.

Think about it this way- the dimension here is dropout spacing/width. The locknuts touch the inside of the dropouts, the axle protrudes into the fork of the dropout.

In the hub end, this width is often referred to as Over Locknut Dimension or O.L.D.

Of course Sheldon has a page about it- https://www.sheldonbrown.com/frame-spacing.html

If the OP is measuring the thru-axle proper...it is easily possible it measures 145mm in overall length. While rear and front spacings on the frame are standard....the thru-axle length itself is anything but standard.

All About Thru Axles - Fairwheel Bikes Blog


And that is not an inclusive list.

It's not a thru axle it's a hollow axle with a bog standard quick release. He's just measuring the axle length, not the locknut spacing.

For a standard quick release axle, the axle would be about 10mm longer than the OLD distance.

So, a 145mm axle would be for a 135mm OLD frame.

There are some tandems that use 145mm OLD hubs.