A more thorough explanation:

HORIZONTAL UNION

A horizontal union is a type of union that represents workers who share the same skills or profession, regardless of the industry they work in. These unions are also known as craft unions.

Examples of horizontal unions include the International Brotherhood of Electrical Workers (IBEW) and the United Association of Plumbers and Pipefitters. Both of these unions represent workers who have specialized skills in their respective trades, but they may work in a variety of industries such as construction, manufacturing, or maintenance.

Horizontal unions are formed to represent workers who have similar skills or professions. These unions allow workers to come together to negotiate better wages, benefits, and working conditions. The IBEW and United Association of Plumbers and Pipefitters are examples of horizontal unions because they represent workers who have specialized skills in their respective trades, but they may work in different industries. For example, a plumber who works for a construction company and a plumber who works for a manufacturing company may both be represented by the same union.