13. Slope refers to the degree of inclination of a diagonal line towards a horizontal line, represented by the symbol ∠. When labeling, the direction of inclination of the symbol should be consistent with the direction of inclination of the marked slope.

14. The types of dimensions in the assembly drawing include ① specification dimensions, ② assembly dimensions, ③ installation dimensions, ④ external dimensions, and ⑤ other important dimensions.

15. The symbol "∠ 1:10" indicates a slope of 1:10, and the symbol "1:5" indicates a taper of 1:5.

16. Line segments in planar graphics can be divided into three types: known line segments, intermediate line segments, and connecting line segments. Their drawing sequence should be to first draw known line segments, then draw intermediate line segments, and finally draw connecting line segments.

17. A line segment with known fixed and positioning dimensions is called a known line segment; A line segment with fixed dimensions but incomplete positioning dimensions is called a middle line segment; A line segment with only fixed dimensions but no positioning dimensions is called a connecting line segment.

18. The projection surface where the main view is located is called the orthographic projection surface, abbreviated as the front, represented by the letter V. The projection plane where the top view is located is called the horizontal projection plane, abbreviated as the horizontal plane, represented by the letter H. The projection surface where the left view is located is called the side projection surface, abbreviated as the side, represented by the letter W.