La gestión de proyectos de triángulo de hierro tiene unas restricciones que se consideran "de hierro" porque no se puede cambiar una de ellas sin que afecte a las demás. La gestión de proyectos de triángulo de hierro original, propuesta por el Dr. Martin Barnes en 1969, sigue un enfoque en cascada para el desarrollo de productos: el alcance es fijo y los recursos y el tiempo son variables. Para un equipo de software, esto significaría que los equipos comienzan un proyecto definiendo los requisitos del producto para determinar el alcance del proyecto (una lista de elementos de trabajo). Los recursos y la planificación son variables y se estiman en función del alcance fijo.

El propósito de la gestión de proyectos de triángulo de hierro es dar a los equipos de productos la información necesaria para hacer compensaciones que ayuden al negocio. Por ejemplo, si los equipos se enfrentan a un alcance fijo, pueden estar a mitad de un proyecto y darse cuenta de que no llegarán a la fecha de publicación. Las únicas variables con las que pueden jugar son: 1) Tiempo: pueden aceptar una fecha de publicación posterior o 2) Recursos: pueden añadir a algunas personas más al proyecto, lo que aumentará los costes. A medida que el desarrollo de software ha evolucionado en el siglo XXI, la necesidad de una mejor colaboración y la capacidad de responder rápidamente a los comentarios de los clientes se ha hecho fundamental, y para ello se creó la metodología ágil.

If your team practices waterfall project management or are new to agile development, the important thing to remember is the difference between what is fixed and what is estimated. Unlike waterfall development, agile projects have a fixed schedule and resources while the scope varies. While the scope of a project might change in agile development, teams commit to fixed iterations of work: sprints if you're using a scrum framework and WIP limits if you're using a kanban framework. It's also a best practice to keep teams fixed throughout the development process. By keeping teams consistent on a product or project, they become more efficient through developed trust and continuity.

The idea of scope is the same in agile development: what software to build and deliver. However, agile focuses on high-level requirements rather than trying to come with deep and detailed requirements upfront. The scope of a project gets regularly managed and groomed (prioritized) by the product manager in a tool like Jira. The product manager decides which work should be accomplished in the next sprint based on agile qualitative and quantitative feedback from various channels (market conditions, customer feedback, competitions, etc..). And because resources and time are fixed, it's easier for development teams to react to market changes and to deliver value to customers faster. This transparency of constraints keeps teams honest about a consistent and fast release cadence, which is a key tenant of agile development; and by looking at projects through the lens of the project management triangle teams are able to adapt without abandoning a plan.

> > Set up your first agile project with this interactive tutorial

As projects become bigger, more teams are needed and the time box gets longer. Thus, the notion of fixing resources and time, while scope varies, is not a valid approach for all agile projects. Long-term agile planning requires a more flexible project management triangle that allows teams to plan ahead and ensures that they're meeting the business objectives. Think for instance about the lean startup movement, and the notion of a minimum viable product (MVP). An MVP by definition is a small set of features (scope) that delivers customer value. To get to that MVP, teams might need to stick to a fixed scope – the number of features – with time being their only variable (e.g. you can't release without certain features, so the release date gets pushed). Only after launching the MVP, teams switch to a variable scope.

Regardless of the differences between waterfall and agile development, when using the project triangle, there’s no right or wrong way. It's there to help you make the best decisions and trade-offs to reach your business goals. A tool like Timelines visualizes the building blocks of a plan – scope, people, and time – to help teams plan in real-time. You can easily play with scope, teams, and time to plan your next product release, using the team's existing data in Jira.