> ## Documentation Index
> Fetch the complete documentation index at: https://docs.glidepathcpm.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Understanding Dependencies

> Dependency types, lag, lead, and how relationships drive your schedule

# Understanding Dependencies

Dependencies define the order tasks must occur in. GlidePath supports all four standard CPM relationship types and allows you to fine-tune timing with lag and lead values.

## Feature Spec Reference

**Module 4.1 -- Relationship Types**

## The Four Dependency Types

### Finish-to-Start (FS)

The most common relationship. Task B cannot start until Task A finishes.

**Example:** Site survey must finish before foundation excavation can start.

### Start-to-Start (SS)

Task B cannot start until Task A starts. Both tasks can proceed in parallel once the predecessor begins.

**Example:** Concrete pour and curing monitoring start at the same time.

### Finish-to-Finish (FF)

Task B cannot finish until Task A finishes. Both tasks end together.

**Example:** Commissioning documentation must finish when commissioning itself finishes.

### Start-to-Finish (SF)

The rarest relationship. Task B cannot finish until Task A starts. Rarely used in construction but useful in just-in-time scheduling.

**Example:** New shift cannot finish until the replacement shift starts.

## Dependency Summary Table

| Type             | Abbreviation | Meaning                     |
| ---------------- | ------------ | --------------------------- |
| Finish-to-Start  | FS           | B starts after A finishes   |
| Start-to-Start   | SS           | B starts after A starts     |
| Finish-to-Finish | FF           | B finishes after A finishes |
| Start-to-Finish  | SF           | B finishes after A starts   |

## Lag and Lead

### Lag (Positive Delay)

A lag adds a waiting period between the predecessor and successor.

**Example:** FS+3d means the successor starts 3 days after the predecessor finishes. Useful for curing times, delivery lead times, or approval wait periods.

### Lead (Negative Lag)

A lead allows overlap between the predecessor and successor by entering a negative lag.

**Example:** FS-2d means the successor can start 2 days before the predecessor finishes. Useful when preparatory work can begin before the predecessor completes.

### Entering Lag/Lead

In the dependency picker:

1. Set the relationship type (FS, SS, FF, SF)
2. Enter a number in the **Lag** field (positive for delay, negative for lead)
3. Select the unit: **Days**, **Hours**, or **%** (percentage of predecessor duration)

## How Dependencies Drive CPM

GlidePath's CPM engine recalculates the schedule every time a dependency is added, changed, or removed:

* **Forward pass:** Calculates the earliest start and finish for each task, respecting all predecessor relationships and lag.
* **Backward pass:** Calculates the latest start and finish without delaying the project end.
* **Float/slack:** The difference between earliest and latest dates. Tasks with zero float are on the critical path.

## Screenshot

<img src="https://mintlify.s3.us-west-1.amazonaws.com/glide-path/assets/screenshots/03-01-visual-diagram-or-gantt.png" alt="Dependency Types Diagram" />

*The four CPM dependency types shown with visual examples.*

***

## Next Steps

* [Creating Dependencies](/dependencies/02-creating-dependencies)
* [Critical Path](/dependencies/04-critical-path)
* [CPM Fundamentals](/scheduling/01-cpm-fundamentals)