Check your progress relative to your reference line frequently. And check for square frequently with a combinationsquare.
刨的过程中要经常根据刚才画的参考线检查刨削的进度，并用组合直角尺检查是否方正。
2
This combination is named the square of the "interval".
人们把这个组合称作“间隔”的平方。
3
But the actual record, 4.17m square km in 2007, was the product of an unusual combination of sunny days, cloudless skies and warm currents flowing up from mid-latitudes.