en.wikipedia.org/wiki/Horizontal_line_test

In mathematics, the horizontal line test is a test used to determine whether a
function is injective (i.e., one-to-one). Contents. [hide]. 1 In calculus; 2 In set
theory ...

www.virtualnerd.com/algebra-1/linear-equation-analysis/intercept/horizontal-vertical-lines/horizontal-line-definition

Note: Ever look at the horizon when the sun is rising or setting? Know why it's
called the horizon? It's a horizontal line! And just like the horizon, horizontal lines
...

www.mathopenref.com/coordhorizontal.html

A horizontal line is one the goes left-to-right, parallel to the x-axis of the
coordinate plane. All points on the line will have the same y-coordinate. In the
figure ...

www.mathopenref.com/horizontal.html

In geometry, a horizontal line is one which runs from left to right across the page.
It comes from the word 'horizon', in the sense that horizontal lines are parallel to ...

www.mathsteacher.com.au/year8/ch15_graphs/05_hor/ver.htm

Plotting a horizontal line and plotting a vertical line.

www.coolmath.com/algebra/08-lines/08-graphing-horizontal-vertical-lines-01

Horizontal and Vertical Lines. What if we want to graph this? y = -2. Hmm... It
doesn't look like there's enough there! Where's the x? We're used to these: Ax +
By ...

www.khanacademy.org/math/algebra/two-var-linear-equations/hor-and-ver-lines-alg1/e/horizontal-and-vertical-lines

Practice your knowledge of horizontal and vertical lines: their graphical
presentation, their slopes, and their equations.

study.com/academy/lesson/what-are-horizontal-lines-definition-equations-quiz.html

In this lesson, we will learn about horizontal lines. What properties are unique to
horizontal lines? How do you recognize the graph or equation...

mathforum.org/cgraph/cslope/horizontal.html

Horizontal and Vertical Lines. Lines that never slant up or down are called
horizontal lines. Here is a picture of some horizontal lines: ...

digital-photography-school.com/using-horizontal-lines-in-photography/

There's something about a horizontal line in an image that conveys a message of
'stability' or even 'rest'. Horizons, fallen trees, oceans, sleeping people – all of ...