Likely as a result of investors’ leaning toward defensive assets late in 2018, the iShares 20+ Year Treasury Bond ETF (NASDAQ: TLT) jumped 6.17% in December.

TLT’s December bullishness has the exchange traded fund residing 2.49% above its 200-day moving average and just 4.41% below its 52-week high. The $8.45 billion TLT, which tracks the ICE U.S. Treasury 20+ Year Bond Index, has an effective duration of 17.38 years, meaning the fund is highly sensitive to changes in interest rates.

Short-term Treasury yields usually rise along with investor expectations for tighter Fed rate policies while longer-term yields are more sensitive to growth and inflation sentiment. Investors typically monitor the yield curve, or difference between long- and short-term yields, as an indicator for economic growth. The last time short-term rates exceeded long-term yields was before each recession since at least 1975, or something also known as an inverted yield curve.

TLT’s December rally sets up some notable technical scenarios.