Danger This Valve Must Remain Open Tag
Accident Prevention Tags are available in two materials:  

Rigid Vinyl tags are highly durable and resistant to moisture, grease and corrosion.  
The matte finish accommodates writing with a pen or pencil. 
Vinyl tags have metal eyelets. 
Price per Pack of 10 Duplicate Tags - 3 1/8" x 5 5/8"
 
Heavy Duty Cardstock tags are best for indoor usage, yet extremely strong.
Ideal for writing on with a pen, pencil or marker.
Price per Pack of 50 Duplicate Tags - 4" x 8"

For custom tags see APTC