A line can be leased as either a 2-wire (1-pair) line or a 4-wire (2-pair) line. A 2-pair line uses 1 pair to transmit and 1 pair to receive. With a 1-pair line, you transmit and receive over the same pair of wires.
perpendicular line
Line segment
I suppose they could be called a pair, just like two horizontal lines could be called a pair, but it's not a "mathematical" or "technical" term.
The edge of a tetrahedron is the line where any pair of its faces meet. Equally, it is the line joining any pair of its vertices.
A line can be leased as either a 2-wire (1-pair) line or a 4-wire (2-pair) line. A 2-pair line uses 1 pair to transmit and 1 pair to receive. With a 1-pair line, you transmit and receive over the same pair of wires.
Which pair. Of line is parallel fghj
There is no specific name - other than what you used: "a pair of parallel line segments".
line up
perpendicular line
Line segment
A pair of perpendicular line segments is not shown among the pictures you've submitted.
I suppose they could be called a pair, just like two horizontal lines could be called a pair, but it's not a "mathematical" or "technical" term.
The edge of a tetrahedron is the line where any pair of its faces meet. Equally, it is the line joining any pair of its vertices.
1 pair of parallel line
A cross is a pair of intersecting lines.
A plus sign (+) would be a pair of perpendicular line segments.