Straight Edge and Bubble

I scrambled up the cedar rib... two careful, breathless pulls. On the upper shelf, someone had left an unpainted wooden folding ruler resting flat, its brass-pinned joints snapped out into a long straight beam. Right beside it lay a single clear glass marble. My breath caught—was it rolling away? I dropped to both knees, clutching my green die against my chest. The marble was dead still, resting in a natural groove of the cedar grain. Using both gloved hands, I gently nudged the ruler flush against the marble until they met with a soft tick. Then I slid my die right into the corner where the wood edge meets the curve. Everything locked in a steady tangent. No tremors... completely parallel.