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Intermediate · 2 min read

Build and verify logic in P6

Create predecessor and successor relationships, inspect driving ties, control lags, and eliminate unsupported open ends.

Working definition

Logic is the executable sequence of the plan. P6 supports Finish-to-Start, Start-to-Start, Finish-to-Finish, and Start-to-Finish relationships with lag. The goal is not to maximize relationship count; it is to model the real handoffs that determine when work can begin and finish.

Version scope

Desktop P6 Professional workflow. Menu labels are aligned with Oracle’s current Version 26 help; older 24.x deployments may show minor wording or privilege differences.

Key takeaways

  • Relationships must represent real execution handoffs.
  • Overlapping work often needs both start and finish controls.
  • Review driving logic, open ends, lags, constraints, and external relationships together.
01

Use the relationship that describes the handoff

Finish-to-Start is common but not universal. Overlapping production may need Start-to-Start plus a finish condition. Finish-to-Finish logic may need a separate start driver. Start-to-Finish is rare and requires explicit explanation.

02

Trace the driver, not just the row

Use Activity Details, the Gantt relationship lines, and Trace Logic to identify the relationship actually driving a date. Then inspect calendar, lag, constraint, and external-project status.

Add and verify activity relationships

Use Activity Details for precise entry and Trace Logic for validation.

Menu path: Project > Activities > Activity Details > Relationships

Before you start

  • Defined work sequence
  • Activity IDs and WBS complete
  • Approved lag policy

  1. Select the successor

    Choose the activity whose start or finish needs a driver and open Predecessors or Relationships.

    Verify: The selected activity ID matches the intended handoff.

  2. Add the predecessor

    Assign the predecessor, relationship type, and only a supported lag.

    Verify: The relationship describes the actual operational condition.

  3. Check both ends

    Inspect predecessor successors and successor predecessors, or show relationship lines in the Gantt.

    Verify: The same tie is visible from both records and points in the intended direction.

  4. Trace the path

    Open Trace Logic and follow driving and non-driving relationships around the activity.

    Verify: The activity has a real start driver and its finish drives the next required work.

  5. Schedule and challenge

    Run F9 and inspect changes in dates, float, criticality, and open-end checks.

    Verify: The result matches the intended sequence and does not rely on accidental constraints.

    Field-ready control

    Logic quality check

    • Real predecessor handoff
    • Real successor handoff
    • Relationship type explained
    • Lag minimized and supported
    • No unintended external link
    • Driving relationship identified
    • Schedule recalculated and log retained

    Common questions

    Can I draw relationships directly on the Gantt?

    Yes. Oracle also supports predecessor, successor, and relationship tabs. For professional review, verify the resulting IDs, direction, type, and lag in the details rather than relying on the line alone.

    Sources and scope

    Follow the reference trail.

    This guide is original educational commentary. Kairos reference records identify the source organizations, editions and scope behind this guidance. The original publishers retain their publications and rights.

    RelationshipsOracle · checked 2026-08-01Adding relationships between activitiesOracle · checked 2026-08-01Primavera P6 Professional User Guide, Version 26Oracle · checked 2026-08-01Schedule Assessment Guide: Best Practices for Project SchedulesU.S. Government Accountability Office · checked 2026-08-01