
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.

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Johnson Level & Tool 40-0912 Self-Leveling Cross-Line Laser Level, Red, 6 x 7 Inches
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π§ Swanson Speed Square Layout Tool & Blue Book Combo Pack: Perfect for Precision Measurements & Layouts with Bonus Combination Square Included!
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β Stainless Steel L-Square, Size 3-Inch x 4-Inch, Zona 37-434
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π CAMWAY 6 Inch Digital Caliper with LCD Display, Electronic Vernier Caliper, 150mm Stainless Steel Feeler Gauge, Plastic Digital Caliper Set for Zero Setting Inside/Outside Diameter, Depth, and Step Measurement Tool
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