Can I use a yardage book or GPS app during my round?

Yes, you can use a yardage book or a GPS app that gives you distances and basic course layout. Rule 4.3a allows distance and directional information, but you can't use anything that measures elevation changes, recommends a line of play, or picks a club for you based on where your ball sits right now.

What the rule actually lets you do

Rule 4.3a says you may use equipment to get distance or direction information — that includes a laser rangefinder, a GPS watch, a phone app with a course map, or a good old paper yardage book. You can also use notes you made before the round, like club selections from previous rounds or swing thoughts you wrote down. Recording stats during the round for later review is fine too; you just can't process that data mid-round to help you decide what to hit.

Where the line gets drawn

The rule draws a hard line at anything that "artificially eliminates or reduces the need for a skill or judgment essential to the challenge of the game." That means no elevation readings, no slope-adjusted distances, no "plays like" yardages, and no feature that tells you where to aim or which club to hit based on your current position. Green-reading maps are allowed only up to a certain size and scale (the limit in Clarification 4.3a/1); a committee can ban them entirely with Model Local Rule G-12. Using your phone as a level or a plumb line to gauge slope is also prohibited.

Penalties if you cross it

First breach in a round: two strokes in stroke play, loss of hole in match play. Second breach: disqualification. It doesn't matter whether you meant to gain an advantage — if the feature exists on your device and you use it, you're in breach. The safest play: turn off slope, elevation, and club-recommendation features before you tee off, and treat your yardage book or app as a reference for raw distances and layout only.

Exceptions that change the answer

• A committee adopts Model Local Rule G-5 → Distance-measuring devices of any kind can be prohibited for that competition.
• A committee adopts Model Local Rule G-12 → All green-reading materials (paper or digital) can be banned for that competition.
• You use information gathered before the round → Previous-round notes, swing tips, and club recommendations from past play are always allowed.

Frequently asked questions

Can I use a GPS watch that shows distances to hazards and the green?

Yes. Pure distance information — front, middle, back of green, carry over a bunker — is explicitly allowed under Rule 4.3a(1). Just make sure slope and elevation features are turned off.

Is a paper yardage book with green contours legal?

Yes, as long as it meets the size and scale limits in Clarification 4.3a/1. A committee can still ban all green-reading materials with Model Local Rule G-12, so check the local rules sheet.

What about an app that tells me which club to hit based on my GPS location?

That's not allowed. Interpreting distance information to recommend a club or line of play crosses the line into prohibited use under Rule 4.3a(1).

Can I check wind speed on my phone during the round?

You can check a weather forecast for wind speed, but you cannot measure actual wind speed at the course with an app or device. That's prohibited under Rule 4.3a(2).

What if I accidentally leave slope mode on but don't look at it?

If the feature is available and you make a stroke while it's enabled, you're in breach. Turn it off before the round starts to avoid any issue.