Although they might work sometimes. . The seams of the bubbles will get stuck or the glass in the plastic is NOT flat so you can count on it. Instead, I used a self-leveling laser. The laser hit the string straight and I didn't look back.
π Kapro 905-41-24 24-Inch Level Tool - Premium Bubble Level with Shockproof End Caps, Superior Accuracy & Visibility, Optivision Red and Plumb Site Dual View Vial, Condor Spirit Level
10 Review
Johnson Level & Tool 40-0912 Self-Leveling Cross-Line Laser Level, Red, 6 x 7 Inches
10 Review
π΅ IRWIN Tools Bullseye Level (1794487) - Blue
9 Review
Laser level HiLDA 4D/16 set 4
44 Review
Triangle Group TR918 225/45 R18 95V summer
61 Review
Kapro - 353 Professional Ledge-It Try & Mitre Square - For Leveling And Measuring - Features Stainless Steel Blade, Retractable Ledge, And Etched Ruler Markings - 12 Inch
23 Review
β Stainless Steel L-Square, Size 3-Inch x 4-Inch, Zona 37-434
8 Review
π Johnson Level and Tool 400EM-S 12-Inch Heavy Duty Metal Combination Square for Professionals, with Inch/Metric Measurements, Silver - 1 Square
8 Review