Chigadee London

Hat hair or helmet head is a no no in RL but in SL it is a must if you want to look great wearing a hat. Unlike RL, great fitting hats are not a matter of getting the hat to fit the hair, but rather, the hair to fit the hat. Here is some great tips I wrote up to help us all in SL.
HOW TO MAKE HAT HAIR
1. Make a copy of your hair and rename by adding Hat Hair to the name of the hair
2. Wear the hat hair copy and a desired hat (preferably one of mine J)
3. Right click on hair, choose edit
4. Check box - Edit linked parts
5. Check box – Stretch. * Make sure Stretch Both Sides is NOT checked
6. Right click individual prim hair
7. Decrease size of prim using the appropriate x, y or z until it disappears inside the hat
8. Repeat for all hair sticking outside of hat
9. If you make a mistake on an individual prim, right click the prim and in the Edit Menu above the screen, click Undo.
10. Be patient since it may take a few tries to get it right. It is really worth it to have that perfect look when wearing a hat.

Matty Luminos

I made some hats in SL and I ended up putting an instruction notecard in with every hat, explaining how to do this.

Individual hair prims can also be moved, or even made transparent if moving/resizing proves difficult.

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meow.