--- /dev/null
+/*
+ * Jalview - A Sequence Alignment Editor and Viewer ($$Version-Rel$$)
+ * Copyright (C) $$Year-Rel$$ The Jalview Authors
+ *
+ * This file is part of Jalview.
+ *
+ * Jalview is free software: you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation, either version 3
+ * of the License, or (at your option) any later version.
+ *
+ * Jalview is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty
+ * of MERCHANTABILITY or FITNESS FOR A PARTICULAR
+ * PURPOSE. See the GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with Jalview. If not, see <http://www.gnu.org/licenses/>.
+ * The Jalview Authors are detailed in the 'AUTHORS' file.
+ */
+package jalview.api.analysis;
+
+/**
+ * An interface that describes classes that can compute similarity (aka
+ * substitution) scores for pairs of residues
+ */
+public interface PairwiseScoreModelI
+{
+ /**
+ * Answers a similarity score between two sequence characters (for
+ * substitution of the first by the second). Typically the highest scores are
+ * for identity, and the lowest for substitution of a residue by one with very
+ * different properties.
+ *
+ * @param c
+ * @param d
+ * @return
+ */
+ abstract public float getPairwiseScore(char c, char d);
+ // TODO make this static when Java 8
+
+}