On a sectional chart, a Military Training Route (MTR) is depicted as:

Why →MTRs are depicted as thin gray (or purple, depending on chart series) lines labeled with 'IR' (Instrument Route) or 'VR' (Visual Route) followed by a route number. Military aircraft may operate at high speeds along these routes, often below 10,000 feet. Per AIM 3-5-2, a route number with 4 digits (e.g., VR1207) means no segment of the route is above 1,500 feet AGL; a number with 3 digits (e.g., IR206) means at least one segment is above 1,500 feet AGL.

The trap →MTR lines can be easy to miss visually because they do not bound an area. Pilots who only look for airspace boundaries overlook them. The digit rule is also easy to reverse: more digits does not mean higher altitude.

Field note →A 4-digit MTR number = entirely at or below 1,500 AGL, squarely in low-altitude airspace. A 3-digit number = at least one segment above 1,500 AGL, and other segments may still be low. Either way, treat an MTR crossing your area as a live hazard and check its activity times.